Because there's no alternative service to choose from.
Raindrop.io is aggressively priced for all that it provides (I have switched from pinboard to this, and it is a near 1:1 replacement). In fact, it provides features such as highlighting / annotation that Pinboard.in will never implement.
Wallabag, Linkding, and Archivebox are three selfhosted options that come to mind, that serve similar purposes.
The best part of pinboard is that there is none of this... stuff.
Once I finally got around to trying it, I found it to be a performant webapp. More performant than most.
The home page really doesn't present it in a good light, but the actual service is good, you should give it a shot.