well from my understanding the staff responsible for seat allocation have specifica instructions to reserve particular seat rows and positions for particularly suitable people. it so happens that the best leg room seats are actually right next to the escape exits (no seats in front). the staff are instructed to wait until they can see people who look willing and physically able ot help people out the emergency exit and even open the exit door in an emergency. if you arrive too early, the staff may put you elsewhere to hold on to the chance that a good candidate will arrive.
so don't feel bad. but yea, leg room is horrible on todays airlines.