The reason is to do with physics and the engineering of drivers. To reproduce low frequencies at standard levels in a room a driver needs to be large to displace enough air. For high fidelity in the home this tends to be around 10-12" or several smaller drivers. As the frequency rises the cone is going to cease moving as one and start resonating which degrades sound quality. A driver can typically cover a frequency range of about a decade with a flat response between a low frequency roll-off in output due to being too small and a high frequency degradation due to being too big and the cone, surround and suspension resonating. In order for the resonances to start at a higher frequency the cone needs to be smaller. Fortunately at higher frequencies physics requires less air to be displaced to be equally as loud as low frequencies. So a smaller driver that doesn’t reproduce the low frequencies enables us to cover another decade at higher frequency.

This leads to a high sound quality speaker requiring around 3 driver sizes to cover the 3 decades from 20-20kHz. Other details to do with overlapping crossover regions, smooth radiation patterns, controlling room modes at low frequencies leads to the optimum being more like 4 in the form of subs distributed around the room for the lowest couple of octaves and 3 way mains.

Having said that a small and modestly priced 2 way with 6.5" midwoofer and 1" tweeter can often do a pretty good job. It won’t play loud enough cleanly, go deep enough cleanly and will have some audible degradation due to resonances but often not to an intrusive extent.