Because the sodding things just keep duplicating and replacing whatever you put on the mannequin.
Seriously, this was broken from time to time in Hjerim (one character could change mannequin equipment just fine, one was stuck with one locked as a Whiterun Guard) and it's a MUCH bigger problem in Hearthfire. Two of my Lakeview Mannequins are now basically useless. The Falmer one I managed to sort of fix, providing I ignore the fact the boots permenantly switch to Fur boots now. The Imperial one is stuck on light armour, and will duplicate infinitely if I try and remove/swap it.
While I appreciate the potential free gold exploit, I REALLY hate that this bug is still here. It's bad enough that the trophy room animals are permenant with no option to change them (stupidly enough...) without having the supposedly changeable option bollocksed as well! ??