have never had a problem stowing my rolling carryon personally
You may not have but on a full flight, there's just not enough room when everyone and their hamster brings one on board. Those after you might have struggled (especially if you were able to preboard and/or get on early). When we had to gate-check bags, those were the first ones we grabbed so anyone who insists on bringing a rolly on board know this, in all fairness!
Also with children, it's better to have your things at your feet. Easier than standing up, pulling the whole darned thing down, unzipping... Just not convenient especially on really long flights. Better to use a tote or backpack and stuff it in front of the seat in front of you-or even better, in front of the child's seat (since they don't really need the legroom).
Of course, often for those of you who have the luxury of flying with the other parent, one can bring a rolly and the other a backpack. Just organize your things in mind with the possibility of having to "give up" the rolly bag.
Since most airlines are charging for checked bags domestically these days, people want to bring as much as possible on board that they can. Just keep in mind that it might not all fit and don't put your lap top or any other delicate and/or necessary item at risk because last minute, some F/A grabs your bag. I always asked "Computer? Camera? medications??" before I sent it forward but not all will.