Why is it when you friendzone someone, they think you don't want to be friends?

You seem to be the one who doesn't want to prioritize the feelings of a friend with romantic intentions towards you. That's fine—you're under no obligation to return such feelings, and it's better to be honest about your lack of interest rather than lead them on. As you say, life happens, and you can't always focus on one person.

However, if you'd like to maintain some form of friendship—or at least part ways on good terms—there are considerate ways to go about it. Here are some tips on how to let someone down gently:

Be direct, but also compassionate. Let them know you value their friendship, but only see them platonically.
Drop subtle hints, like referring to them as a sibling or hinting at other romantic interests.
Never meet one-on-one; always include other people in your meetings to avoid any potential awkward situations.
Only hang out with them as part of a group.
Don't touch them or flirt with them inappropriately, and make sure your interactions remain unflirtatious.
Don't lead them on. Don't give them the attention that a romantic partner would receive.
Be honest about your feelings, and tell them that you don't want to damage your friendship by pursuing a romantic relationship.

Remember, though, that being friend-zoned can be a frustrating and painful experience. It's not uncommon for the rejected party to feel humiliated, especially if they've been leading themselves to believe that their feelings would eventually be reciprocated.

The internet is filled with advice on how to escape the friendzone, but many of the tips out there involve manipulating or deceiving the other person, which is hardly cool. As several sources note, the best way to go about it is to prioritize their comfort and respect their boundaries. Give them space to process their feelings, and let them decide whether they can handle being friends without romantic expectations.

Ultimately, if you're unable to be a true friend and value their feelings, it might be best to part ways—at least for a while.

Maybe they haven't quite accepted that they can't get everything they wanted from you (are still upset that you are not romantically interested), and are dissatisfied with the amount of your attention they have. I think some people have a hard time understanding that just because they want someone's time and attention, does not mean they are owed it or entitled to it.
I hope it works out.

Dont feel bad about knowing what you want ajd setting boundries but also make sure you are not sending mixed signals

Maybe it's more like embarassment on her part.