Preference really. But most of the front-enders I work with seem to prefer Sass for some reason and thus have been pretty dedicated to Foundation in the past. With this official port being introduced it makes bootstrap a more viable option to those who spend most of their time using Sass.

In addition to what has been said by others, the Rails asset pipeline supports Sass out of the box. Now Rails developers can use Bootstrap without bringing in third-party LESS support into Rails.

Official support from the Bootstrap team rather than third-party support. Sass and LESS releases will be hand-in-hand now rather than staggered.

Short answer; preference.

Using Bootstrap pretty much meant that you had to use LESS, now you can choose between Sass and LESS. The differences are not that big, though.

Seems to me Sass is preferred because of Compass tools.

You can copy existing CSS into a SASS project using SCSS.