Probably because they opted for a smaller cone with more excursion to reduce cabinet volume and hence increase WAF and lower production and shipping costs. It's cheaper to build a higher powered amp to give more excursion than use a lower powered amp with a larger cone area. It's easier to manage distortion with a smaller diameter cone as well. Ian has always said he's sacrificed some SPL to optimize linearity. With the v4 sealed subs, he's nailed it.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated