'Well, they are different, they are large, and they need a lot of power..'
Yes, can be, not necessarily. Single driver full range OB designs are neither large nor inefficient. I think the main reason for the lack of popularity is esthetics. They "look" too simple and too crude for most people and WAF is below the line. They are also not very versatile and sound best only with certain type of music and certain upstream gear. I just recently built a pair for the first time using the Tang Band W8-1808 full range drivers and they sound real nice but only with jazz, blues, folk vocal type of music and at moderate volumes. Could be my amplifiers since all I have are high powered SS amps.