Why regtest network is absent in btcwallet?

[ahstroomnetwork]

As far as i can understand support of regtest is implemented in btcd which btcwallet is intended to use with.
But according to the title, is there any reasons of not implementing regtest in here?

[Roasbeef]

IIRC, simnet was created around the original time regtest was. It doesn't have a block limit, persists to disk across restarts, and also does difficulty retargeting.

PRs accepted to add regtest natively.