Time constraints
I've spent thousands of man-hours writing code in VB6.0. I've consumed years doing it. I just don't have enough time to re-write it. In fact, I don't want to. I don't care if VB2008 is free, because backward compatibility was more important to me than the introduction of a "genuine object oriented programming language".
Seriously, how would microsoft employees like to re-write all their programs again. All of them. Sit down and type them all again. Work out all the bugs again. Idiots.