I'm also an experienced VSRG player. I've been playing since ~2002, and have played both Stepmania and O2Jam before osu!mania. I've been playing osu!mania since day 1 of the public release as well, so I've seen just how the community has changed and grown over time.
The same things Shoegazer said apply to 7k/8k as well. Both O2Jam (7k) and LR2 (8k) have heaps of absurdly difficult charts which offer a variety of different kinds of patterns for top tier players to push their skill ceiling with.
O2Jam has a wide variety of charts involving patterns such as nearly unpassable mashes, teleport notes, inverted charts (all LN), etc. These patterns are all effectively unrankable for various reasons in o!m, which means few mappers are willing to invest the time into making charts of similar difficulty, and the majority of players will never see these maps.
Similarly LR2 has a huge selection of insanely difficult charts with a wide variety of patterns, although far less emphasis on LNs than O2Jam. There is however an entire difficulty scale dedicated to LNs, and there are LN Dans, but the majorty of difficult LR2 charts have few or no LNs. Of course, just because they don't have LNs doesn't make them easy. I'd say many or most extremely difficult LR2 charts would also be unrankable in o!m (although they'd have a better chance than O2 charts).
Because of this, and other issues with community management, the entire ranking process as a whole, and the issues with o!m's mechanics, many experienced players don't see osu!mania as a desirable game to invest time in.