My thoughts for what its worth.
With the average mature adult male 6' to 7', and females upwards of 8'+ they are an impressive size / bulky snake but one that can be handled by most adults (although I would always advocate the normal safety measures of having someone on hand if you struggle).
I've owned three "normal" boa's in 30+ years. By that I mean common, rather than dwarf / island species. Each one has had their own personality. One a male I simply never trusted full stop. Another a female she was more trust worthy than some dogs I've had. My current 8' male is somewhere in between. Once he's been made aware that he's coming out he's fine, but I wouldn't try that at night if he's hungry...
Ignoring morphs for now... the natural patterns and colouration of the tails make this one of the most attractive mid-size snakes. The arrow shaped head, bat-wing saddles, and in some cases perfect red triangles outlined in white on the tails just makes them appealing. Now throw in some genetic enhancements and (in some cases) all the above gets taken to another level.
Put all the above into a smaller more manageable package such as a dwarf island species and you have catered for all the markets. A chunky looking snake, nicely marked, possibly a decent temperament and no larger than a decent Royal python !
These are just some of the many reasons IMO boa's are so cherished by their owners