well I think a hand like 99 would call the flop, but if the turn is a jack it devalues 99 a lot since we could decide to bet a jack on the turn if checked to on the flop and turn

now we have to weigh pps that want to peel a flop vs. hands like Jx that fold the flop but would improve on the turn to second best
there are just not many hands that would improve to second best and want to call the turn other than dominated pairs like T7
there are a lot more hands that have 4-5 outs that we'd rather either fold or call a 1/2 pot bet than draw for free

in a hu pot I would check more than bet because:
we're giving a free card to one person and not two
we're more likely to get a fold from one person than from two

from my db my cbets get called twice as often when it's 3 way than HU