The code is old, and hard to secure. It lives in the browser rather than a subprocess, so is unsandboxed.
The FTP protocol itself isn't a nice binary protocol - it was designed for humans to type by hand, so has a lot of flexibility, leading to a lot of corner cases in the code.
There is also the fact that the flexibility of FTP allows the browser to attack other devices on the local network. For example, I could navigate an iframe to FTP://evil_payload@127.0.0.1:3389, allowing me to send a possible exploit to your your machine, bypassing firewalls.
Considering how few people use it, and the risks it still poses to everyone, I can see why they want to get rid of it.