Well... I could give a thousand reasons really...
To start with, in UT99, you can go from idea to development to release really quickly, and in a game that you really like to begin with, and that's very empowering and rewarding.
Unlike indie games, you can just focus in creating that weapon you want to create, or that map you want to play, or that trigger you want to use, etc, rather than think up the whole gameplay and do player models and whatnot.
And unlike more recent games, you don't need to spend months to years to develop a high quality mod, and that is when these games actually allow modding to happen, which generally is no longer the case nowadays.
And you're sure to a very good extent that what you develop now will keep working in newer systems well perhaps within a decade at least, since the engine doesn't rely on almost anything from the operating system API itself unlike others, and since Epic released native C++ headers this means that the engine can be extended and even fixed to some extent, and that's why we were able to have things like more modern renderers like OpenGL and D3D9, a fairly good anti-cheat (ACE), fixes and overall improvements (XC_Engine), etc.
And Epic cares enough to actually keep the master server up, and we got other master servers provided by the community as well given that the engine allows for such.
As a consequence of this, this is probably one of the games in the entire world with the most amount of content. Of course there's both good and bad content, but even if you got rid of all the bad one, this would still hold to be true.
Also among all UT versions, UT99 is still the most popular one.
Other reasons are also:
- the nostalgia factor: there's a special attachment to this game from many of us;
- the community is quite good overall;
- the age range is quite big and varied as well, so you can hold more mature discussions most of the time with more mature people;
- the game works well pretty much on any hardware nowadays, and even from a Pen drive, so it's very accessible to anyone;
- even without going native, there's a lot which can be done to make the game even better;
- Unreal has a much better engine nowadays, giving a glimmer of hope of a good upgrade without going overly overboard in the future;
- no DLCs, microtransactions or any risk of getting banned over modding the game;
- the overall visuals and atmosphere are still quite unique to this game and engine in particular, unmatched by anything else;
- there are tons of servers and half of the admins are willing to try new things on them, so there's always opportunity to see your mod played;
- it's just pure fun to see your stuff working in a game that you love.
Of course, this game has also many problems as expected from such an old game, but it just aged so damn well that these problems are often overlooked, and generally have some kind of fix which can be done by the community.