1. Fans were pissed, but mostly fans who played older GTA's. If someone started his "gta career" with GTA4, I'm sure he liked it. For me GTA4 wasn't that bad. It has a long story, large map and more realistic physics, but somehow it's not the same fun as in GTA3 or VC, where stunting with a car or bike costed you some HP or blown vehicle, whereas in GTA4 you are dead . But it wasn't bad.
For me, the main reasons for lack of interest:
1. Less updates. Really. Recently I wanted to download a tool for disk defrag, I searched the best and the recently updated one. When I see that a program was last updated 5 years ago, it means to me that it is discontinued.
2. The games. As I said the future belongs to new games, because potential new players choose them.
3. No promotion.
Please tell me SugarD, how many of players that participated in the MTA R2 beta test were unknown for you? I think you know all of them. They came to see what's up with the good old MTA VC. But how many players does MTA R2 has in average during the week? Okay...
Plus, IV:MP is an unofficial mod, because GTA4 has got a build-in MP mode. It sucks from what I've heard, but doesn't matter .
Many of them were unknown. In fact, many of them I didn't even know, and I know almost all the old veterans and MTA Developers, past and present.
To add to this, there are actually new people who are from the MTA:SA/SA:MP-era that found GTA3:MTA, MTA:VC, and VC:MP to be incredible fun. Two players I can think of specifically, Jaysds1 and Callum, each came from that specific gaming era, having not even played GTA III or VC before, let alone GTA3:MTA, MTA:VC, or VC:MP. They are now in love with it, and one of the is a Developer for my team, whereas the other is learning programming languages so he can eventually become one. Callum even expanded his former community to contain MTA:VC servers, and revived their VC:MP server, when they previously only had active MTA:SA and SA:MP ones.
Again, you cannot judge it based on how old a game is. Yes, statistically-speaking the older the game, the less popular it will be over time. That does not mean it won't be popular at all, nor does it mean that no one wants to play it. Remember, these mods are for the people who enjoy them. That is why they are made, and that is why they are free. The GTA modding community has always been this way over the last 11 or so years because we are all fans with a common desire. That has never had anything to do with playerbases, nor should it ever. This is not a product we are selling, so who cares if it has less players than a newer GTA MP mod does? The point is to make it for people to enjoy, not to purchase.