Is there any reason why this hasn't been implemented?
Not to sound like a dick, but this is one of the most basic features present on almost every website or ui driven by user content (including Youtube). It's been a standard feature of every file browser since at least 1990. I'm sure it's almost definitely been discussed behind closed doors, but the fact that it still has not been implemented means that this feature either can't be implemented, or (like the inability to filter reposts) has intentionally not been implemented.
One would think that the biggest priority would be ensuring that users can find GOOD music as quickly and easily as possible. But there is virtually no way to sift through the overwhelming majority of bad music to get to the hidden gems, even my feed which was carefully curated before the introduction of reposts is a total crapshoot.