It was a combination of factors, really:

• Not often, but often enough...I had some significant problems with version upgrades and would end up having to configure the whole system again from scratch;
• My hardware was getting pretty old. While pricing out an upgrade for it, I was surprised to find a more capable Synology for less (DS1821+);
• Remote backups. I have had a bazillion different solutions for off-site backups, and none of them have stood the test of time. The last one before Synology was a separate machine running Duplicati, pulling data from TrueNAS, and sending it off to an ODROID-HC2 running Minio. It worked sometimes. Hyper Backup on Synology is an entirely different world of ease and reliability--it has been flawless.
• I kind of got tired of TrueNAS as a hobby. I ran into and waited for resolution on bugs, had a bunch of customizations done from the command line that I'd forget about before the next time a version upgrade wouldn't work and I'd have to start over, and just in general had to keep up my knowledge managing the thing (cruising the forum and subreddit). Synology isn't as flexible, but I never have to deal with it...and that has been pretty nice.