Why do aerobatic aircraft have separate tanks instead of a single acrotank?
A long time ago aerobatic aircraft had just a single tank with a flop tube inside it.

This works "OK", but when you're doing something complicated with a half-empty or mostly-empty tank, it's too easy for the flop tube to ingest air instead of fuel.  This leads to performance problems with both the engine and the pilot.

The solution is to have a very small "acrotank" with a flop tube and an arrangement of fuel line plumbing such that the tank remains 90% full, 90% of the time.

In my Laser, there's a very large tube (2" to 3" diameter) that feeds from the bottom of the main tank to a tube at the top of the acro tank (with that tube extending inside the tank to near the bottom).  There's a small vent/return line.

When I'm right-side-up, the acro tank fills from the main tank in seconds.  When I'm inverted, the tube arrangement prevents most of the fuel from returning to the main tank.  Some returns via the vent/return line, but I still have about 5 minutes of sustained inverted flight fuel.