Maybe because when you pronounce a B, it's very hard to make it sound sharp. The lips rolling off one another naturally have a gentle ramp-up. It's really hard to have a sudden transient. You can record this in a free DAW and see it in front of you.

By contrast, it's very hard to make a K- sound without a sharp transient.

With B you are adding pressure to two soft tissues in contact.

With K you are adding pressure when one tissue is hard, and saliva is present (causing a relatively sudden production of the sound).

Try making B sound sharp, hard to do while saying Bouba.

Try making K sound soft, basically impossible, especially while saying Kiki.