That's what's known as the "I don't want to do it price". I have done cross platform code in CUDA before so I know what's involved. Plus you still pay more in electricity than you earn in coins mining with Nvidia GPUs, so it's a less extreme version of what happens with CPU mining.
I'm totally for keeping CGMiner as simple as possible. I actually wonder if one day you will drop GPU support for strictly ASIC support. However, the higher performance of native cuda seems to be pushing the cards into the realm of slightly profitable. A 580 seems to push 280KH/s, which is about $1.25/day profit.