This is actually because of the obvious reason (for once in surfing) - length of the board and rocker (the curve of the board at the nose).
A wave about to break is curved, so if you try to fit a longboard in the curve it might just be too long. Secondly, LBs tend to have lower rocker which also means they dont fit in the 'curve' as well as a shortboard - the SB follows the curve of the wave, the LB doesnt.
There is a way 'around' it, which is much much easier to describe than to do. If your LB is at an angle to the wave when you pop up or just after you pop up, then you can 'fit into' the wave because you are going across the curve.
So you either have to paddle across the wave at enough angle to not get stuck in the curve but not too much so that you cannot catch the wave (this technique may or may not work depending on the wave) or, more likely to work but harder to do, pop up and turn the board almost simultaneously ie paddle into the wave normally but pop up really quickly and turn immediately. SBers use this latter technique as well, but they can often turn much easier/quicker than LBs, so its a relatively easier manouvere that can be done quicker and with a greater success rate.
Anyway, the benefits of being on an LB are catching the wave early (have more time to pop up) and you get to catch the wave before it even gets to the shortboarders, so you can 'steal' their waves in full compliance with surfing ethics (more or less). So no need to do late takeoffs.