According to CarAdvice, the reason is because a turbocharged small engine is unable to fulfill the shortage of torque from low engine speeds. And to counter that, BMW M chief, Frank Van Meel said that electrification is needed which in turns means adding more weight.
"I wouldn’t do a four-cylinder standalone turbocharged with high performance, because you always have the characteristic that if you want high performance you lose the low-end torque and you lose the overall drivability you want to have from the car." explained Van Meel.
“With the current generation we see ‘E’ motors that are still not strong enough for M applications, and if you look at plug-in hybrids, it will add 2-300kg - which, for a car like an M3/M4 with 1500kg - would put that completely out of balance and we couldn’t re-balance that towards a typical M philosophy", he added.