Commitment isn't necessarily about being exclusive nor is it about doing something without question. It's about being willing to take the time and effort to make something happen even when obstacles get in the way. If someone has a career they want to pursue that takes almost all of their time then a relationship isn't really in the equation. You can only commit yourself to so many things before you run out of a finite amount of time and energy. Once you go beyond your physical ability then you'll inevitably start reneging on your commitments. I think of it as doing what you say and saying what you'll do. If you start saying things and not doing them then you're unwilling or unable to commit and should stop saying you're going to do things. In the extreme you've become liar and are untrustworthy.
Time and energy are finite resources to our reality. It requires forethought and planning to fit in all the things we want to do, like our hobbies or our careers, and a relationship is yet another thing we want to do. Some things are more important to us than others and there's nothing wrong with that. We also have to recognize when that is the case and selectively include the things that are most important to us and willfully excluding the things that aren't as important to us that may want our time anyway. Forgetting to account for all the trappings that go along with something is a quick way to not being able to do the things we really want to do. In a way this boils down to learning to analyze situations, estimating, and planning.
Relationships are more than just opportune moments where two or more people just so happen to have time available at the same time. They're about committing to planning to making the time and taking the effort.
PS: Anything can be considered a relationship in this way; friends, work, careers, hobbies, families. Any of these can be shorted in the same way. For example, right now I'm shorting my commitment to my freelance work by not setting expectations or planning properly for when things will get done (poor planning, poor time management: poor exclusion of things that want my time, and not giving myself enough time to recuperate after my last gig). In a way by posting this I'm being hypocritical about one thing with another. Regardless, there's still truth here that can be applied to me and to others so that doesn't make the point less and it shouldn't take perfection before one can talk about something (else we'd never have any teachers or any food for thought).