It's actually not, and its because of how the FAA allows carriers to use "standard weights" for weight & balance purposes. Standard weights include you AND your carry-on items too. In the summer, you and your carry on bags weigh 190lbs according to the government. If the overhead bins are full, any bag that has to be gate-checked and put into the belly of the airplane, now has to be "accounted for" to the tune of 30 lbs per bag. I know, it seems asinine, because your 22" rollerboard is still on the airplane whether its in the bin above your head, or in the cargo hold, but the minute the bag goes into the cargo hold, it now counts as more weight.

The agents know the flight is payload optimized, so they are trying to prevent having to gate-check bags which just eats into the available payload margin. Or, think of it like this: every 6 gate checked bags equates to about 1 SDSB passenger who is trying to get on the flight!