good question but here is my reasoning not sure if it is the case but I
think it has to do with mowers now having mulch capiblites (the wider the
mower the more grass and the harder to mulch properly) . It may also be
with the longer the blade the faster the blade tip speed is thus you have to
set the max rpm of the engine lower to keep withing standards (blade tip
speed is set by standards for safety) so a 21" blade can have higher rpm
then a 22" blade to keep the blade tip speed within standards. the higher
the rpms the faster the inner part of the blade spins where it cuts the
grass 2nd & 3rd times for mulching so again it helps with that. So as i said
not sure if that is that is why it also may be that 21" mowers are
manufacture top selling models and that is why.