Cost cutting, pure and simple. It's more expensive to hit sensitivity targets with 8ohm speakers than it is by 'cheating' with 4ohm because of the extra motor force needed, and more compliant suspensions means tighter tolerances required. Both those mean more cost. So instead of spending the bucks, some manufacturers will just parallel drivers to halve the impedance and give a more attractive sensitivity spec.
8 ohm speakers are IMO a must for many of the low-feedback tube amps out there. Into 4 ohms you can see +/-2db swings in the frequency response. What really can be a problem is when some manufacturers call the speaker 4 ohm, but in actuality you see dips to 2 ohm and below. That will give any tube amp fits.