To add to points made about weight restriction....

Relatively recently, the FAA raised the average passenger weight... I believe 10-20 pounds depending on the season... and baggage weight. When doing weight and balance for a flight, everybody weighs the same, and bags weigh the same (for the most part). With the rise in average weight, a full load of people in the old days probably weighs the same as two or three people fewer. Thus, a full flight is under tighter weight restrictions. Add to that bad weather en route or at the destination and increased fuel necessary and they can't fill each seat.

At American Eagle, things got so restrictive after the weight increase they stopped selling the last row of 4 seats on the Saabs and actually yanked that row out on many airplanes. No way to take a full load of people and take any bags at all. Even a full load of passengers would have put some aircraft over the take-off weight.

I've done a lot of flying in/out of STL on American Connection the last few years... almost all on ERJ. Frequently their STL-PHL flights on a 50-seat RJ can only take 42 folks, especially when there's bad weather in Philadelphia and they need extra holding fuel. What a fun afternoon before Thanksgiving when they're telling a plane of people booked to about 53 with all shown up that 11 are not going to make it, and some peoples luggage still wont be going.