Honestly, it's hard to articulate or even pinpoint one thing. I like the layout and options of FreshRSS, and a few little things; for example, I use categories, and when I hit 'mark as read' on a category in tt-rss, it would indeed mark everything as read, including items that had refreshed behind the scenes since the last time I opened the category, effectively marking items as read that I hadn't seen yet. FreshRSS seems smart enough to realize that those items haven't been displayed.

Also, tt-rss' developer, fox, is rather abrasive, akin to Theo de Raadt from OpenBSD (which I love and will always use, despite Theo's abrasiveness). I will admit, the projects do generally benefit from this unyielding approach, but it makes it a chore to request help, knowing that you'll likely be upbraided for asking questions.

I also like that FreshRSS has better 3rd party app support (at least on Android).