You get more power than you need from the engines already with the simpler increase in capacity.
LOTS of power doesn't work in a small bike as it flexes too much, the trailing arm is too short etc.
Sudden increases in power is death on a motorbike on anything but simple straight roads - and even then it's iffy. So until recent times turbos were only for the idiots. More recently some have taken turbos as aftermarket - the 'busa being the normal one as it's big and heavy and strong enough to cope and isn't' an agile sportsbike.
Smaller engines dont' return as large benefits as bigger engines so it's easer to add another 250cc to the engine to get h power than a turbo to a 250cc
"A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'