My personal opinion:
It's successful (I say "successful" in the same way I'd refer to it as "popular") because of its appeal to beginners, of which there are a lot of, especially since the whole F2P thing.
The layout is incredibly easy to learn since everything is right in your fucking face from the get go - it's the most basic type of gamemode that allows respawning - and while there are no flanking routes, there's so much (dispersed) action going on it's still possible for spies and pyros and other shorter-range classes to go in for the kill (as well as anyone being able to slink on by) while everyone else is distracted. Basically, IT WORKS SOMEHOW.
Because there's so much breathing room in the map, it ends up being a training-ground/combat-arena hybrid map, which appeals to new players who may not want to play against bots but still want a casual atmosphere to learn the ropes of the game at their own pace while getting to grips with rocket jumping, juggling, airblasting and all those other techniques you see the cool and handsome people do regularly. And surprisingly, while there may be quite a few snipers on the map, there rarely ever is a genuine sniper-fest going on (at least during the times I've played on it, which was loooong ago mind you).
Also, because there's literally nothing more than a few brushes and one or two props to render at once, there are no framerate issues for anyone. This lack of content also means it's got a very small file size and players can download the map quickly from servers.
In my opinion orangex is only a good map when you see it from the eyes of a beginner and nobody else (except perhaps those who prefer trading and events sessions to actual gameplay). It breaks many mapping rules such as having huge sightlines, having little height variation (except for that tower in the center of the map), no detailing whatsoever and so on - mapping wise it's a bad map - but really, it's not even designed for normal play; it's a fun map for dicking around and being silly on the occasion, as well as for newbies to practice in a semi-controlled environment and anyone who wants to play in a more laid-back atmosphere that official maps just don't have.
In short, orangex is good for beginners and anyone who enjoys not playing seriously and to everyone else it can eat a dick for all they care. I don't like orangex, but I don't hate it either.
Anyway, just my two cents.