Well, look at the math. Seat pitch today runs about 33in. Actually less, but it makes the math easier. If a row had 3 seats in it, that would mean 3 bags to fit in 33in, or 11in per bag. Even wheels first, the bag width is 14in maximum, meaning you are 6 in short every row. Multiply that by 22 rows, you end up with 132in per side short. That's over 9 bags a side! And that does not take into account space lost to bin side wall and other stuff stored in the overhead. It's actually pretty amazing that they so few bags end up having to get gate checked.

And every inch that they reduce seat pitch means another 2 or 3 bags per side have to go into the hold. Those edge loading bins suddenly start to look really good.