Xenocide is in development hell, but UFO:AI is still working. Do you use SVN? Because you can check-out developmental builds using that.
Indeed, they could probably have used a more suitable engine when they begun, but they've done too much work using it to change engines now (it'd be essentially throwing away all work on the project up to now and starting over).

I didn't get the SVN yet, they make it hard to find and I'm not THAT interested in playing it. Each demo I tried just doesn't feel fun, the mechanics they use are old and dated and it's not aging well as it develops. This is me of course, I'm sure many people think it's better than XCom.
We might need a reboot that doesn't suck like UFO: ET did. If I learn to program in C++ (got the books, not the time) then I might get to making something like that. But it's a dream for now. The market for TBS games is a limited and focused one, not many companies want to risk it anymore and that sucks for TBS fans.

Just thought I'd chime in with one thing... UFO: ET unmodded, is a mess. However it's been modded so extensively after release by fans of the original X-Com series, so that with those mods installed it really does capture the spirit of X-Com in my opinion.
So if you have UFO: ET, I'd say give Greyfiend's Unimod an install, and see what you think... I love it, personally.
http://www.ufogr.com/ufoet/eng/history.html#ver_3_60_03
And getting that link for you guys, I see he's updated it today... so... haha... i'm off to update and shoot some aliens.