I guess it’s because of low base damage (128%). So essentially it gives only a nice debuff for DA which AFAIK doesn’t stack with other sources of DA debuff. So if you already have some means to lower enemy DA you don’t need it at all, better invest this SP in other skills.
There are a couple of reasons why MA is not popular for 2h builds:
It is a singletarget procc (unless you use a specific unique, wich lets it procc on multiple targets, but that unique is part of a shield-focused set). 2h usually doesn’t have an issue with singletarget.
Lack of points. In order to let MA deal more damage than an hit with your regular attack, be it Cadence, Savagery or sth else, it needs a few points of investment. It’s not a good onepointwonder, you want to get it at least to lvl 7-8. And, as rmsinj has pointed out, the %weapondamage doesn’t increase with investment. So there is a very high chance that you will have to put even more points into it, to make it worthwhile.
The sexappeal of MA is the DA reduction, but you can get that from Blindeside (Blitz) in an AoE.
That being said, if you have the points for it, per se there is nothing wrong with investing into MA with a 2h.
Animation duration + low %weapon damage means that you get nothing in return for a MA proc.
In fact the more you gather attack speed the more your attack flow will stutter due to MA animation.
Every WpS serves a function and is best used in different scenarios. More WpS does not always make a build stronger, be it 1h. DW. 2h, and ranged versions of the three.
I didn’t know it has a long animation. But comparing it with Belgo, I just decided to switch to the latter just to try cuz they are about the same in terms of damage. Overall feeling it as a damage improvement.