(explanation of why older members stopped using ultra dex)
Basically, most of the older members don't even use it anymore because the Ultra Dex has been very inaccurate in the listing of moves. These included missing of certain moves for specific mons when they learn it (eg. Mamoswine learning Double Hit by level-up), listing of TMs when a Pokemon doesn't even learn it (eg. Venonat learning Aerial Ace TM; it only learns it when it evolves into Venomoth), errors in BM/MT/SM. The last, which happens very commonly. For many Pokemon, a certain MT would be missing from the list (normally Platinum MTs, not all though). Therefore, it became annoying that when people use the Ultra Dex for updating their stats or to find moves for battles, they would look through an inaccurate Ultra Dex. With online Pokedex available like Serebii/Veekun, it became more common for people to stop using Ultra Dex instead.
(analysis of other online pokedexes)
K so, I am pretty sure Jack made it because the URPG works differently from the games. We allow the usage of every move if you've bought it, and little to none online Pokedex actually offer all of the detail in a page. Smogon is an very accurate site for move details, but you have to click the G/S/C tab to look out for any G/S/C specific TM that the Pokemon might not get in R/S/E, like perhaps Zap Cannon (some mons don't get it in D/P/P, but do in G/S/C, so when you view it in Smogon, which shows D/P/P as default, it won't show Zap Cannon). Not only that, not every single SM are listed. They don't group their moves by Level-Up, TM, HM, BM, MT, or SM as well. Serebii, people tend to use it alot but I fail to see the good point of it. It shows pre-level up moves which is useful, but like Smogon, it doesn't list previous version-specific TM, nor SMs. People have also reported that Serebii was missing some lvl-up moves, though I don't know about that. Then lastly, we have Veekun (which is the best imo). The bad side is that Veekun recently received an upgrade and the SMs are only shown on the old version. Adding on, not all SMs are fully listed. That's about all for the bad sides. People have reported that Veekun was inaccurate since it was listing moves that a mon doesn't learn, like Magnemite learning Icy Wind, though it is because it gets that in G/S/C, which means Icy Wind is a legal move for Magnemite.
When I started out playing URPG, I've been using the Ultra Dex ever since, I was a little annoyed that it wasn't that accurate after I became a little more "experienced" in the game. Ever since, I've moved onto Veekun. Though, the advantage of looking through a page and seeing all the moves that a mon can learn, even SMs, was no longer possible. From like February this year, I've already been working on the Ultra Dex for an entire correction of moves. So basically, I look through every mon, check their Level-Up moves (formatted it to level learned), check their TMs (formatted it to the TM number they were assigned in game, since Jack did it, but when additional edits were made, they became scrambled up instead), and their BM/MT/SMs are now fully corrected. I am proud to say that the Ultra Dex is probably 99.9% accurate now. =3 Back to my point, I believe the Ultra Dex is now truly Ultra in the sense that it is probably the only online Pokedex that offers all the information within a page for easier access, and the only one that has all of the assembled SMs (took me a while to connect several sites to create a full list of SMs), and the only one that doesn't list "repeated moves". Like for example, if Walrein can learn Ice Beam naturally, it won't be listed in the TMs then, which is one of the reasons that I like about the Ultra Dex, Jack started it though (not all were removed again).