It's not just as simple as commitment level or whether or not someone "wants to take the band seriously." As people go through varying stages of their lives, many different things take priority. Work, kids, sports, church, etc. all come and go to varying degrees as we move through stages of life.
One good example is that when we hit the 30s, some of my friends who played music and ball were ready to quit playing ball, so suddenly they had time do take the band thing more seriously, and I just wasn't ready to quit playing ball so I could not be in the band. When another group of guys I play with all became empty nesters 10-12 years ago, they too had time to play music 3 nights a week. I was still playing ball.
Some guys have jobs that interfere, kids in school that play sports, etc., etc. It's not always "how seriously they take music" that makes someone a good band mate. It's how "available" they are. A single guy with an 8-5 job can practice/gig 4 nights a week and get away with it if he wants. I know I've told this before, but I was in a band and committed to 1 night per week practice, and 2, maybe 3 weekend gigs per month... assuming they'd all be local. Then they sprung it on me that they were playing a 3 day music festival that was over 1000 miles away... I would have needed months advance notice to plan for that.