Because we need to validate moves on client side,
and only standard chess is implemented there.
Feel free to improve:
https://github.com/ornicar/lila/blob/master/ui/round/src/plugins/sanWriter.js
Some variants have same move generation,
and have therefore been included:
chess960, kingOfTheHill, threeCheck.
Horde could be included if moving first rank pawn
by two squares was implemented.
a05181c784