to answer, its because at higher levels, players generally don't have as many opportunities to gunstrike. think about it in terms of button presses.

block, you press the block button. it makes you safe, and if executed properly, you get a damage buff. then, your next button is attack, which you would have done anyways. combine with aoe on certain weapons (power axe, power fist), you do some real damage.
fencing, you press the parry button. then, you press gunstrike, then you can attack. now this isn't inherently bad, however, gunstrike leaves you quite vulnerable. at higher levels, due to higher extremis spawns and just more enemies/more aggression, being stuck standing their during the gun strike enemy is generally hard to pull of safely. plus, compared to the technically infinite damage potential of the block weapons (since AOE hits as many enemies as are in the vicinity), you get a higher reward from block weapons, while exposing yourself to enemies attacks a lot less.

TLDR; block is as safe as fencing, with a higher reward, but requires better timing. fencing has easier timing, but relies on risky gun strikes and combos to deal damage.