Errrr, I don't think zone battle is not played because it's too challenging. Surely everyone agrees that getting used to ZB takes a lot less time than getting used to racing ? Well that's my personal experience anyway. You can't really say it's challenging online either, considering how few games are hosted. If it's not from people I know, most of the ZB rooms are hosted by hurr-durr noobs who put some stupid setup like syncopia @30 or something. Reminds me of that one time when that guy set up a Sebenco Climb @40 to scare me off of the room... that was cute.
Though the lack and weapons+ the fact it's not a mainstream mode (ie something major in the offline campaign or the reason why you buy the game to begin with) explains the lack of popularity methinks.

I don't think they're too long. I don't remember when the zone decreasing thing start, but it's at a reasonable time imo. Definitely not too hard to get to 55. On any clear track that's easily done under the time limit. If anything, I'd say it's a lot easier than classical racing or zone : you can recover from mistakes much more easily in general (wall damage is negligible, health easily restored,etc) and goals more straightforward (all you have to do is chase around speed pads, health management for BRs is inexistant, position of others on the track and lines are mostly irrelevant).

I just think that people mostly play the campaign, do a couple of ZB against the retarded AI (in retarded settings to top it off. The hardest is what, MH @40 maybe ?), dismiss it as a bogus bonus mode designed to look pretty alongside detonator (which is waaaaaay harder to master imho), and get back to the racing mode.

I'll be able to play some Wipeout starting september, so if you have a plan to manipulate people into playing ZB by then, do enact it^^