I left Xenforo to use Woltlab suite, I didn't have to pay extra for the CMS that comes with Woltlab and it's a bit cheaper than Xenforo. I've also left phpBB to go with vBullein recently because I needed the extra plugins like a group system and blog system for one of my communities. In the past I moved from phpBB over to Invision, but that's because I was young and wanted to use IPB even though I really couldn't afford it back then and I can't now either lol.
I left ProBoards to go back to ZetaBoards due to finding it harder to familiarize myself with the software and I essentially knew everything I could about the ZetaBoards platform, so I figured why reinvent the wheel.
Early on in my forum administration career, after starting using free forum hosting software that used phpBB2, I moved over to using MyBB for a while, before finally settling on phpBB3 as the software I used for the majority of my projects for quite a long time.
I also used MyBB on some projects though, and was an admin/staff on forums using vBulletin and one using SMF. Back in those days, nothing ever felt quite like phpBB3 for me.
Besides a few phpBB forums at first, I quickly settled on InvisionFree. That was back in 2003. While I was on IF, they came up with their own software called ZetaBoards. I was there until they got bought out by CrapaTalk.
I did have a license for IPB since 2011 but once I got familiar with XF since 2015, I’ve been there ever since.
Invision v5 is truly amazing, but something just keeps drawing me back to XF.