First, to qualify what I'm talking about, I marched in the Pride 4 years, 03, 04, 05, 06. I've opened a T or two.
To make the power T formation in a straight line would require each member on the wings to make very very very specific steps off of the yard line. One 8to5 step is 22 1/2 inches. (8 steps per 5 yards), to make a curve like that in a small area would place guys at weird intervals, and would give a high chance of failure/weirdness. You might end up with some straight lines, bad spacing, etc. With that few people spread that far apart, I don't think it would get the desired effect.
Then you have the tradition aspect. The opening of the T has been relatively unchanged in the past 40+ years (save the fact that it used to be east/west rather than north/south out of the endzone. I think there's a much higher chance of screwing up one of the very most exciting moments in college football. This is one of the things people can anticipate all of the off season, and know exactly what they're gonna get.
It's 2am, and I know I've not put things in the most eloquent manner, but what I'm getting at is that is "if it ain't broke, don't fix it"