Why was FieldTrip maintained in SVN and not in Git?
This question does not apply any more.
We started using the CVS version control system for FieldTrip development in 2004. In 2009 we switched to Subversion, motivated by the SPM and EEGLAB teams also using it. Subsequently in 2016 we switched to git and GitHub to facilitate external contributions.
Please see our git and GitHub tutorial for more details.