Why?

In many cultures and in many people's minds - dare I say, most - sex and love are linked. If your starting assumption is that people should only have sex with others that they want to have a long term relationship with, then having a long string of partners indicates a general low level of commitment to any given partner. This would imply, given these cultural assumptions, that this person is not a good bet for a stable, long term relationship. And in cultures where chastity is seen as virtuous, these standards are applied both to men and women (if not entirely equally). A woman who is part of a conservative sect of catholicism, for example, would likely be less interested in a man who was known for sleeping around, rather than a man who has remained a virgin waiting for marriage.

Another perspective is that of fairness and dignity. Suppose OP and OP's love interest are in middle school PE class, where they play pickup soccer. OP is good at soccer. But every day, OP's love interest picks the rich kid over OP, because she knows that the rich kid always has their parents buy their team pizza after school. So OP sits on the bench watching the other kids play every day, and never gets pizza. Then suppose on the last day of class, the coach announces that the winning team in today's soccer match will be guarenteed A's for the semester. Now OP's love interest picks OP over the rich kid. I think it would be quite understandable for OP to be put off by this behavior. They would feel like they haven't been properly valued by their love interest, that they are now being used. I think an argument can be made that OP would be a better paragon of virtue if they let go of these bitter feelings - but at the same time, I think these feelings should be understandable to most people who make an attempt at empathy.

And another perspective, which I think is the strongest argument, is simply that people like what they like and don't like what they don't like. Our intrinsic sexual/emotional desires are largely not malleable. It's time to stop pretending that people's preferences are some kind of moral barometer. Those on the left have been making this argument for years after all. Gay men, for example, are born gay. No amount of conversion therapy changes this. Trans individuals are trans - if this were something they could change, don't you think they would skip the social stigma and extensive medical procedures? Why wouldnt they just take the easy route of simply changing their minds, if their minds were something they could easily change? We could apply the same reasoning to, say, foot fetishists. They don't carry the same stigma as gay or trans people, but there is still a definite stigma. And having a foot fetish entails being turned off by partners with "gross" feet. Life would be easier for them if they didn't have this fetish. They would avoid the stigma, and their dating pool would be significantly increased - so if they had the choice, why would they not simply stop caring about feet? And we can similarly apply this logic to OP. As we can see from the comments in this thread, there is a stigma attached to caring about the number of partners a partner has had. And if you don't care about this, you will have a much larger dating pool and life is easier. So if this were something which was easy to change, wouldn't we expect people to change it?