The reason is that both systems do the same thing...the seatbelt and the
latch system are both tested to sustain over 5000lbs of crash force. The reason they put a weight limit on the LATCH system is b/c it is essentially just a bar that is welded to the frame of the vehicle. When the seat is crash tested, they only use one or the other system...never both at the same time. Putting both systems on the seat could cause too much strain on the carseat and possible cause the seat to break in a crash. So even having a 'back up', the entire seat could break and fail..then either the seatbelt nor the latch system would help.
As for the
Regents--the weight of the seat and the weight of the child combined, the seatbelt needs to be allowed to stretch properly in a crash.
Britax found that the seat, with a child up to 40lbs, performed best and exceeded minimum standards using the long belt path. Then for children 40-80lbs, the seat performed best using the short belt path.
You can definitely tell a huge difference in the seatbelt length/strength from the long belt path to the short belt path. I will admit that when they changed this..I believe it was 2007 (??? prior to that it was use either install method)...I wasn't happy. I felt that the short belt install didn't give that tight, secure install that I was able to get easily with the long belt install. But after understanding what occurs during a crash and the way that the seatbelt need to stretch in order to ride with the child and slow the child down in a crash, I can see why they did change it.
HTH!