I suppose some people are jamming RUG because they don't want to/can't afford to build other decks. For me though, playing RUG is
not a matter of card availability. I can build plenty of Legacy decks, including all variations of Delver and BUG, UWr Miracles, Reanimator, etc. I also have the cards to build Maverick, Jund/Junk, D&T, etc.
I think there's actually an advantage to being 'under the radar' and being perceived by other players as 'irrelevant', 'not Tier 1', 'out-dated', etc. Obviously I think their dismissals are wrong, but in any case it works to our advantage in that they are not sideboarding as heavily against RUG and/or are not expecting cards like Stifle and are more likely to be taken by surprise. Being the 'underdog' means you aren't facing as much direct resistance in terms of card selection and/or play-testing preparation from your opponents.
Decks aren't really built or played in a vacuum; I consider RUG Delver a good choice because of what is being played right now. I think RUG (at least with the right configuration) has the best matchup against UWr Miracles compared to any other Delver deck. Sure, the other versions have their own advantages, but I think the combination of Nimble Mongoose, Stifle, Bolts for reach and sideboard options makes us well-situated against the format's current 'Best Deck'. So until/unless a card(s) get banned from Miracles and knocks that deck down a notch or two, RUG will continue to be a good foil to the format's biggest problem in addition to being well-positioned against the various combo decks being played right now.
The one factor I would say that hurts RUG moreso that other Delver builds is its relatively bad matchup against D&T -- but this is true of the other Delver decks too. D&T is popular enough to modify your deck to beat, but unfortunately the standard builds running Mongoose + Tarmogoyf gets nailed hardest by Rest in Peace and also needs to draw removal in a timely fashion to deal with stuff like Mother of Runes + Serra Avenger etc. So I think stuff like
Sulfur Elemental is warranted (while also being not-awful against Miracles and combo by virtue of being an Instant-speed, un-counterable threat that ignores graveyard hate).
That all said, playing some other version of Delver (BUG, Grixis, 4-C, straight UR, or even UWR) is a completely justifiable choice. They each have their own advantages and disadvantages. While Deathrite Shaman *is* a busted card, I personally don't think there is one version of Delver that is clearly far and away better than the others. I think that Grixis' popularity and perceived 'success' has more to do with the 'sheep' effect moreso than it being 'superior' to the other Delver decks -- by this I mean that if it performs well in one tournament, more players are willing to net-deck the winning list, and thus more and more players are playing it and thus more copies of Grixis Delver get registered instead of other versions of Delver, and so it is simply more statistically likely that Grixis Delver will continue the trend of being the Tier 1 Delver deck being played. It's a positive feedback loop: more Grixis players (and it *is* a good deck) = more Grixis top 8 results.