Name recognition, for one.
A proven track record, for another, and less likelihood to shut down. Many smaller instances legit did shut down over the years - like I recall one I was wondering if I should join, dmv.social, that chose to shut down during the waves of CSAM attacks, just prior to the automated tool that dbzero created to save the Fediverse.
Smaller instances may also have lesser hardware and less technically-capable admins maintaining it.
I recommend PieFed rather than Lemmy tbh - it just has so many cool features that Lemmy lacks, like categories of communities and combining comments across all cross-posts, so cool!
But if sticking with Lemmy, for anyone in the USA I cannot recommend highly enough Discuss.Online - it has fantastic uptime and a very friendly admin group, both of which are somewhat rare so it’s really kinda odd why that instance has so few users on it (although understandable when most people choose Lemmy.World or lemm.ee, or coming from the EU choose a different regional instance, which is also cool:-).