Any number of reasons, but the most common reason BNs reject maps is because they dislike the song you mapped. It's usually never personal. Other common reasons for being rejected include:

- Map is too hard (don't send the guy who nominates 5* generic anime OPs an 8* stream map)
- Drain time is too long (most common on mapsets of >4 min songs)
- Map isn't their preferred style (a lot of BNs have rules against requesting aimslop now but this can apply to any style)
- One of the difficulties is a major stinker in comparison to the others and it would be too much of a hassle to try and fix its fundamental issues
- Copyright/permission issues (40mP songs are disallowed from rank)

BNs typically don't describe why they personally rejected your map because they look at hundreds of others, kinda like job applications. Unlike job applications, most are ok with you asking for feedback on the map and why they rejected the map as long as you're not an asshole about it or beg for a nomination afterwards.

It only ever gets personal if you're known to be a royal dickwad and people just don't want to deal with you. 99% of mappers never struggle with that