Its probably just because im so used to COD - i've played all the COD's from COD3 - MW2 a ridiculous amount. But as for reasons why i prefer it, well the pace of COD is a lot faster. Yes it's less tactical, and even on 'team' games, its more lone wolf than relying any sort of team tactics, but i like this. As you pointed out, the graphics are better.
I found on the BC2 demo, it was a lot of spawn - run for 5 minutes to where some fighting was going on, kill a few people, die, back to spawn. Unless you were a sniper, which i didn't like either, as, if i remember correctly, there was no breathing action which to me just seemed stupid.
I think in conclusion, COD is just a lot faster paced and requires quicker thinking. Yes on BC, it was possible to be more tactical and work as a team, but you still get wankers who jump straight into a tank and power off to the opposition base.