Many things, too many to even remember.
Very bad SQL implementation is a good start, still bad replication support (compared to Postgres), various bugs present for too long…
https://www.sql-workbench.eu/dbms_comparison.html this comparison is a bit out of date, but explains a lot
Postgres is far superior in every way.
We used MySQL (and Percona XtraDB) servers at work, and it is so bad. So I made several presentations showing generic and specific reasons why Postgres is better. I had to cut a lot of content because MySQL is just that bad.
Some things may not seem relevant now, but as you keep the DB around long enough, you will appreciate the whole package of Postgres.
Things that will help a lot, but are extensions:
There is a DB comparison matrix, but hasn’t been updated in over a year, which is a shame, but still gives you the idea of how different databases support SQL features: link.
Spoiler: postgres has the most support, with a huge lead
Cracked software still enables their dominance over the market.