My young co-worker (girl) broke up with her boyfriend due to the fact that he was not "manly" enough. I saw it coming... even though she did not.
He was a gentle, kind, soft spoken man
She is a pretty, smart, flirty girl. (with no idea what she wants)
Neither are, in any way, abnormal or weird. However, the lack of "Alpha" behavior on his part ruined the relationship. Had he behaved just a bit differently, been a leader, a bit "tougher" or rough around the edges and they'd be married right now.
I'm an old man, and have years of observations and experience under my belt. I don't subscribe to the theory that women know best. As mentioned above, often they don't know what they want and are attracted to 1) money, 2) bad boys, 3) nice cars, 4) airplanes, 5) business success, 6) humor, 7) big muscles etc.
The reason for this attraction is primal in nature. A strong man can work hard. A rich man can afford a family, A bad-boy can provide safety, A nice car signifies the finer things in life, riches, luxury, vacations etc. An intelligent man has a much better chance of success.
Women are often instinctual about such things. My first serious girlfriend loved my gorgeous, red, 1966 Mustang road race car. It was adventure and money for her. My wife liked my college degree, my intelligence (such as it is, hahaha) and saw potential in me.
I have no idea what happened in this relationship the OP talks about. If the guy involved is introspective enough, he "may" be able to determine why.