[gtaylor]

There's a great reason. Browsers are advancing, and it makes sense to gradually take advantage of features as most of the mainstream browsers start supporting them.

The hacks required to make older versions behave tend to pile up with time, increasing complexity and maintenance overhead for the developers. There gets to be a point where the complexity and overhead is no longer worthwhile to appease a very slim demographic.

Just use older Bootstrap versions if you want older browser support. I don't want your legacy users slowing down the progression for the rest of the modern world :)

[dangrossman]

This 'very slim demographic' is more than 1 in 20 people on the web still, and catering to them has typically required very little (maybe a few extra lines of CSS; otherwise, modern websites work fine on IE7, just without the nice rounded corners and such). I'll be using Bootstrap 2.x until that statistic drops much lower.