Typically it's a cooling limitation, the engine will overheat if operated at full power for too long. 100% for take-off, 75% or less for cruising is fairly typical.
In this case there's a little something extra going on, however.
Because the CarbonCub is a 'Light Sport Aircraft' category plane, they had to be super explicit in the specifications that the
'maximum cruise power' was only 80hp or so (
), and that you should never, ever, ever,
ever, ever be above that for more than 5 mins.
No matter what your temp gauge says. Because LSAs are the 'more accessible' planes, they cant be to powerful don'tcha know?