You have misunderstood me. It's not like it needs more strength to carry xbow but to load it. You are pulling the string without additional help of any mechanism so it should require more strength.
You have to consider that xbow guys reload their weapons while standing still, which means they are using their feet to hold the xbow on the ground and then pull up the string with both hands. Its not that you can run around reloading it.
Grumbs: Dedicated xbows are not infantry. For properly using a 2slot xbow you need at least 140wpf to have reliable accuracy. 150-160 or above is probably the number that people use for these weapons. Leaves 1 slot for either a 2h mace, langes messer or any 1h.
Another thing is that people who play xbow might be good at melee because they played infantry a lot. But that is individual skill. A veteran with 5PS could beat a tincan noob with a wooden stick.