For me, personally, it's two reasons.
1) The majority of them are/were castoffs from the bantamweight division who we were suddenly supposed to accept as being top, top fighters. 10lbs is absolutely fuck all, if 10lbs made a massive difference Lesnar and Carwin would have dominated HW until they retired. They didn't, they were beaten by guys weighing more than 10lbs less than them through being superior fighters.
2) The majority of them were/are boring as fuck. During the flyweight division's 'heyday' I was sick of hearing Rogan shilling for the division by constantly claiming they were 'super technical' and suggesting that anyone who found their lack of finishing ability was a fake fan/clueless. Every card FOTN was a flyweight fight, to try and brainwash fans into accepting it - they then did the same thing with WMMA for a while, but like flyweight, they gave up because they realised they weren't fooling anyone.