1. It's not family-oriented
2. The parks are not as clean
3. The employees are rude
I have been going to Universal since the very first time they opened their doors a couple years back every time I have come to Orlando for visiting WDW and have yet "to not see":
loads of families with children in tow of all ages varying from toddlers to teens enjoying the park rides and attractions, park areas that are always clean along with being well maintained during each visit, and have always encountered very friendly and courteous workers throughout the entire facility whether in gift shops/restaurants/ride operators...etc.
Oddly enough in talking with others, a great portion of the negative feedback people state about their families not visiting Universal are exactly some same things I have personally experienced before at Disney also at one point or another.
But, it seems that Disney is somehow given more creedance or justification throughout their expressions of dissatisfaction in comparison to Universal when I have discussed it with others, also leaving me at a loss at times.