In no particular order, some people have shittier connective tissues than other people. Some tissues respond to stress at different rates than other tissues. Some people do not adequately prepare the supine-side grip, i.e. they don't train it as often as they train the pull, e.g. use of straps, alternation of prone/supine hands during work sets. Some people take steroids, which differentially affect tissues. Some people aren't very good at keeping their supinated elbow straight. Some people are older than other people, and connective tissue composition/quality changes with age. So, it's more complicated a situation than your question implies. It's very seldom the bicep's force production capacity that avulses/ruptures the tendon.