Where are all the 1.5 X Barlows?

Maybe I am dense, but why do we see barlows from 2X to 5X but you gotta hunt for a 1.5X? Seems I am missing something here, especially in my light polluted skies, half of my eyepieces can't even tolerate 2X.

Starzona has a 1.5X piggy back barlow that threads onto 1.25" eyepieces. It appears to be ultra-low profile, like a filter, for about 70.00.
Seibert also sells a 1.5x and he's a AM sponsor.

I guess my real question is why is the 1.5 X barlow held in such low esteem such that they are an unusual item. Is a perceived thing, i.e., doesn't seem like enough difference, or is there some other real reason?