BF2142 engine is little better than BF2, better view distance or something. But we can't use all of code what we have on BF2, and we need to make new models, maps, everything. So cons of this idea are enought to don't do it. In the other hand lots of ppl haven't got BF2142. I guess BF2142 is more or less dead now.
From what I know, the BF2142 engine is superior to BF2's. But BF2 was way more popular.
BF2142 has more weather stuff, could support "legs" on vehicles, auto-cannons (or "companions", as you see in First Strike). Much more, but can't remember . All in all a LOT of very nice things we can't have in BF2 unfortunately.
Battlefield 1942 used Refractor engine 1.0 Battlefield 2 used Refractor engine 2.0 and 2142 used Refractor engine an enhance 2.0 which I would just call 2.5
Basically, 2142 is kinda like a mod for Battlefield 2, with a few engine additions and features.