Changing the install prefix for a project like Homebrew is hard, because directory names frequently get embedded in executables and configuration files. The ARM migration was a convenient time to make the switchover, because everything had to be rebuilt anyway.
Brew uses /opt by default as well (but it wasn't always the case, and it used to symlink into /usr/local too while keeping the actual files in the Cellar).
/opt by default is for M1 and later based Macs. intel based Macs still use /usr/local.