Because, as cglazier wrote, you will lose agility and add weight.
It seems that between 90 and 100 cm are the chosen length for many for riding in waves and chop, a perfect compromise.
Having it longer than 100 (well, actually just longer) when not racing, gives some disadvantages that becomes more and more noticed:
It would have a higher turning radius.
You have to drag further out before you can ride (water depth).
It will be a PITA when jumping and doing tricks - the length thus swing weight increases, so if you want to tweak the foil around when in the air, the shorter strut is WAY better.
On the other hand, shorter than 90 works really bad in waves and chop according to many (and me), as you want to foil without touching down occasionally - thus the range 90 to 100 cm is what is used by almost everybody.