My understanding is that that is just a convention that Bootstrap chose: “default” is white, “primary” is blue, “success” is green, “info” is light blue, “warning” is orange, and “danger” is red.
Yes, blue seems arbitrary, but if we take into account that the success, warning, and danger colors seem logical, there wasn’t many simple, popular colors left that look good on a screen. Blue seems like a good choice.