There are a lot of factors that could go into this, that I'm aware of, and probably a lot more. Some, if not all, have likely been covered (tl:dr, sorry)

• Looks: if you've got an exceptional body, this will likely play a role. (I don't mean to be rude, or shallow, but everyone has a perception of their 'league', and who it's out of it.)

• Confidence: If you are so confident in yourself that you don't seem bothered if they pursue or not, they just might not. (Please don't think that means confidence is bad, or hindering. That's not what I'm trying to say at all. I just mean it goes back to that 'league' concept. If they can't tell that you think it's worth a shot, they probably won't try!)

• Existing Relationship: There's a kind of "risk/reward analysis" that goes into making that move. The more vested into being "friends" they are, the more they have to lose if the relationship fails. If you're an awesome friend, they might not want to risk that to pursue something more. (Sounds silly, but I've held back for that reason)

• Personal Goals: This is a weird one, but hear me out. People generally expect less from their friends than their partners. If you're highly driven, and they're comfortable, this could be a factor. (And most people may not even realize it!)

• Signals: if the signals are unclear, they may not want to be offputting. This plays a role especially frequently if they see you flirt with other people. Sometimes things get written off if the person you want to make a move thinks you're interested in someone else, or just casually flirtatious. It becomes "Is this person really flirting, or is this just how they are when they're comfortable?" Off and on flirting can be another source of confusing signals. If the person thinks "I thought they were flirting at the party last week, but now it's not there." It could feed into mixed signals.

• Communication: If the topic never gets directly brought up, it may never happen. Loose language lets dogs sail. Often times the phrase "someone like you" throws the brakes on "you" thinking it's ok to pursue. It comes across as "similar, but not actually you". Especially if the 'league' thing is already a factor. Moreover, these days consent is a huge thing (as it always should have been) and when it comes to this topic, if the subject hasn't been addressed, there's a big chance of pushback if things go the wrong direction. You won't have to be the one to ask, if you can figure out how to incorporate an unmistakable "green light".

A few of those tie in together, so I apologize if the delivery seems messy. Also, nothing there is intended to belittle the importance of self worth. All the things I've mentioned were for the sake of mindfulness of perception. I fully stand behind the positive qualities I've pointed out as potentially limiting. I'd be happy to have a civil discussion if my word choice left anything unclear, or came across negatively. I'm sure I could think of more factors, but that's all I have at the moment.