On Rhydon vs Golem:
Let's just be objective here. Yes they are very similar (same typing, similar stats and movepools). Let's look at the factual differences (without taking the metagame into account first):
Rhydon is slightly more bulky on both sides (does not matter by how much, bulkier is bulkier), hits harder on the physical side (more significant in terms of XHKOs, but even if that was not the case, hitting harder means hitting harder) and gets >100 HP Substitutes. Those are objectively the advantages of Rhydon.
Golem hits harder on the special side, has 5 more base speed and Explosion. Those are objectively the advantages of Golem.
Now, this is a viability rankings thread. I know it seems confusing at first why two very similar Pokemon are that far apart from each other in a ranking. That is because we are not looking at an on-paper power ranking (in which they'd be closer) but rather ask ourselves for every individual Pokemon the question: "How usable is this Pokemon in general in the RBY OU metagame?" (And in the case of Rhydon and Golem also "What reasons are there to use Rhydon/Golem over the other?")
So let us look at the differences described in the first paragraph in the context of the metagame:
The reasons to use Rhydon over Golem are very apparent: Hitting harder and being more bulky, it's clear on paper and a look at the metagame confirms that it actually makes a differnce, not only on paper. I know this has been done a million times, but once more the most important calculations on Rhydon's advantages:
Offensive:
~guaranteed 2HKO on Chansey*
~(pratically) guaranteed 3HKO on Snorlax*
*will talk about these match-ups further below, especially against varaints carrying Reflect, which Snorlax and Chansey commonly do.
~70% chance to 3HKO Slowbro
~2HKO on Lapras with RS, 3HKO on lapras with EQ
~more damage against Exeggutor: Golem needs to RS x4 to safely KO, Rhydon safely KOs with BS x2 + RS x2
~guaranteed OHKO on Jynx (vs Golem's 50% chance)
~45% chance to KO Tauros with EQ after two Tauros Bodyslams
~18% chance to 2HKO Starmie
~65% chance to 2HKO Victreebel
Defensive:
~48% chance to be 2HKOed by Alakazam Psychic (vs Golem's 78%)
~4% chance to be 2HKOed by Exeggutor Psychic (vs Golem's 26%)
We'll have a look at Substitute's advantages further below.
Let's try to figure out reasons to use Golem over Rhydon:
The special attack is irrelevant. The speed only matters against opposing GolDon, which is not nothing but hardly a reason to really give up the power. The only thing that remains is in fact Explosion.
As many people described, the ground types are there not only to put pressure on the opposing Team by threatening huge physical damage, but also to wall a potential electric type from the opponent. Explosing your Ground type obviously does not help here. Also let's be completetly real here, we are talking about Explosion. Admittedly a good move, but by nature a "hit or miss" move (let's not pretend Boom won't also kill your own Pokemon). Consistency is usually reached throguh other means than relying on "hit or miss" (example: why usually top players won't rely on Wrap to win games). The scenario in which you can just click Explosion and whatever you hit is a win for you is rare and hard to achieve. Not to mention that Golem is not exactly the best choice in RBY if you want a boomer. That is because Golem's Boom is usually "telegraphed" because it is 2HKOed by so many things and also usually Golem does not draw in targets that desperately need to be boomed on to be taken out, especially if you don't know if there is a Zapdos in the back. Other boomers also carry usually more defensive utility than Golem, which adds to them being the better choice. Regardless, I don't want to make Explosion worse than it is, it can be a cruacial move and give you an out to win games and especially further your lead in a game, but consistency is the main issue here, in my opinion.
Combine that with Rhydon's surperiority mentioned above and reasons to use Golem get sparse.
But we are not done yet: We have not even talked about movesets, especially the substitute option. I wanna state that both can theoretically run the move, but Golem has to give up either Body Slam or Rock Slide to do so, and with less reward than Rhydon, because Rhydon's Substitutes live a Seismic Toss, whereas Golem's do not. As opposed to Golem's Explosion, Rhydon's Substitute also agrees with the role of the Ground types, as they are slow but hard physical hitters, which makes them especially effective against paralyzed Teams. Substitute can generate free turns on FP turns that are simply game deciding in some situations while not sacrificing your Pokemon. Don't wanna say Substitute in general is a better move than Explosion, but in this particular case I'm almost inclined to say it kind of is. Also Rhydon is a bit more flexible with movesets actually, possesing options such as Leer or Rest, which Golem is hard pressed to find room for, as stated above.
Finally I wanna talk about the match-ups of the two against both of the defining Pokemon of this metagame: Snorlax and Chansey, more accurately, Reflect Snorlax and Reflect Chansey.
Rhydon has a very high chance to KO Snorlax with 3 EQs if one of them crits through Reflect, which Golem can not claim, but rather needs a crit at the exact right moment or Explode to achieve the KO. Not saying that Rhydon likes the ReflectLax match-up, but it at least can win and if it does it is usually quite game changing.
Reflect Chansey with Seismic Toss is also in Rhydon's favour. Chansey should be paralyzed for both, but in any case Rhydon is likelier to KO the Chansey with a critical hit, despite having the slightly lower CH rate, because it has more attempts due to Substitute surviving the Seismic Toss and 4HKOing in general through Reflect, which makes Chansey have to Soft Boiled more.
Reflect-Normals are simply important match-ups, and Golem has no chance to claim them over Rhydon.
Combine all this and using Golem over Rhydon becomes tough to justify. There surely may be teams that apprecitae Golem's Explosion more than they would appreciate Rhydon, but those are not the norm. (Important thing to note here: using both on the same team is not a good idea, since they stack too many weaknesses, it is bottomline an "either X or Y" decision.)
At the end of the day, Golem was made UU for those reasons, and it makes sense.