Sensitivity.
Take a typical "8 ohm" midwoofer and double them up in parallel and you get 6 dB more. Realistically, more like 3.5 to 4 dB but also drops impedance by half. Problem is that impedance varies with frequency and the same midwoofer will have a minimum impedance, usually around 200 Hz. Some more than others.
The last drivers I used in a project had a minimum impedance of 5.6 ohms. In a MTM that's 2.8 ohms at a frequency that demands current. A 4 ohm tweeter doesn't have that current demand. Found a trick to eliminate that impedance dip but it doubles the cost.
Subwoofers are often 4 ohms because the trade-off for bass extension is sensitivity or size. So, they often use lower impedance to get more sensitivity from smaller drivers in smaller cabinets. Automotive subs are sometimes only 2 ohms. Same applies to woofers. Do you want bass? Do you want reasonably sized cabinets?
There's also the advantage of using smaller value inductors with lower impedance and less resulting phase shift.
Take a typical "8 ohm" midwoofer and double them up in parallel and you get 6 dB more. Realistically, more like 3.5 to 4 dB but also drops impedance by half. Problem is that impedance varies with frequency and the same midwoofer will have a minimum impedance, usually around 200 Hz. Some more than others.
The last drivers I used in a project had a minimum impedance of 5.6 ohms. In a MTM that's 2.8 ohms at a frequency that demands current. A 4 ohm tweeter doesn't have that current demand. Found a trick to eliminate that impedance dip but it doubles the cost.
Subwoofers are often 4 ohms because the trade-off for bass extension is sensitivity or size. So, they often use lower impedance to get more sensitivity from smaller drivers in smaller cabinets. Automotive subs are sometimes only 2 ohms. Same applies to woofers. Do you want bass? Do you want reasonably sized cabinets?
There's also the advantage of using smaller value inductors with lower impedance and less resulting phase shift.