There are many reasons why 2142 was chosen.
1. the BF franchise is the best for MP and this the most popular
2. alot of the Devs had experience on the BF engine
3. we started in BF2, but it became clear that 2142 would be our ticket to having walkers
4. the UT engine could never support the epic battles that we wanted at the time, granted the engine has improved since then, but BF was the best engine at the time
5. despite what people may say (its normally people who "heard" and never actually tried", the BF engine is easier to mod for the normal person aka non programmers
Hope that clears things up 

Someone tell Bandu Ron Paul won't win!