I've been part of other online groups that have moved from one platform to another and each time you lose members along the way. It could be that some are still stuck on the old way, sometimes it's just the inability to visit the community due to an IT restriction or it's that people were inactive members and now that they have to make an effort to rejoin the group they won't.
Personally, the hardest part about using the forum is just not knowing who's who. Over the years, I've gotten accustom to people's names and their email addresses. Yet, the forum allows people to pick a name that they'd like displayed and most people are picking things other than their name or some what they used on TT. Yes, I'll eventually learn to equate majorvices, bluesman, theDarkSide, Thirsty_Monk, etc. with some meaning but for those just starting with the forum it can feel strange and uninviting.