Mainly two reasons.
Acoustic suspension reduces distortion
With acoustic suspension the air trapped in the closed enclosure gives a more linear suspension than the suspension of the driver itself, especially with big excursions of the diaphragm.
A closed enclosure gives the most output below cutoff
A closed enclosure gives the most shallow rolloff below the resonance frequency and is stable when driven below resonance. So, it requires less power to extend the low frequency response and it is easy to correct and to extend the low frequencies.
It is not the resonance frequency that limits the bandwidth at the low end, but the maximum possible displacement. But a lower resonance reduces the power demand to reach a certain low frequency, and makes it more efficient.
This creates a full-range two-way loudspeaker
In combination with current-drive and Linkwitz-transformers it is now possible to realize a full-range two-way system. From below 20 Hz beyond 20 kHz. A song like ‘Thanks to You’ of Boz Scaggs of the album ‘Dig’, a nice song for testing subwoofers, gives no problem whatsoever.