That is in fact the most likely answer. In fact, I rather suspect that the only reason we still have pole beans TO BEGIN with (i.e. why that trait was not rouged out at some point in the beans' domestication) is that the Native people who first did so had a ready made climbing substrate already in place in the form of the cornstalks they were using in their three sisters growing plan. Had they not had corn (or had corn remained the fairly short and thinnish plant it was when it was teosinte) it is likely that ALL (or nearly all) beans would be bush. This seems to be a sort of pattern with legumes. In general, when the choice comes down to a bush or a pole type, bush wins. The only legume crops where pole tends to keep it's edge (the wild forms of most of our domesticated legumes start out as vines, so that is the evolutionary norm) is either where they is not much bush DNA available such as with the common/ English pea, or the Lablab (until I got some in a trade I did not know there WAS such a thing as a bush lablab) or where the crop is treated as a mass commodity which is harvested from the field all in one go, such as many cowpeas and most of the "minor" beans (azuki, mung, mothe, urd, rice etc.*) so it doesn't really matter if they make one huge snarl.
In some cases the choice is so overwhelming that one forgets a pole version even exists. I was unaware of such things as climbing soybeans until the day I had some grow (from some black ones I had bought in Chinatown and thrown out) even though I knew wild soy was a vine. And I'm not sure if I have EVER seen a climbing Fava though (since favas are technically a vetch) there presumably MUST have been one at some point.
* Though my grow out seem to have demonstrated that bush forms of these DO exist.
