Probably something to do with energy transfer using a thin mono line vs level fly line... I personally don't notice much difference. Most of my mends are just pull-backs or using the rod to steer the head.
Thicker coated poly running lines are easier to mend because they are larger in diameter which yields higher surface tension which allows you to impart more load in the road which you transmit to the line while mending.
Another reason poly coated running lines mend easier is that they are a higher mass per linear length which allows you to create more momentum for the same exact motion you impart to it vs a mono running line.
Momentum = Mass x Velocity
The combined effect of the two make it easier.......but still difficult......since both are still pretty thin. I like to use poly coated running lines > .035" as it's what I'm familiar with and the improved mending ability. I'm not a super duper caster where my leader unfolds perfectly and my fly is under tension instantly.....I'm always mending to adjust the speed, clean up my crappy casts and for depth control of my fly.
I'm with Danny though.....most of the time I'm using the pullback drop-down anyway to get under tension quicker...but I still like to throw in a mend or two if need be.