Because constitutional status changes based on thin referendum majorities are such a good idea. ![Roll Eyes](https://talkelections.org/FORUM/Smileys/classic/rolleyes.gif)
For comparison, Alaska voted 83-17 in favor of statehood (the second time, the first time it was close-ish and it fizzled), and Hawaii voted almost unanimously in favor of statehood.
I'm sure the last thing that the pro-statehood majority of Puerto Ricans want to hear is some other Americans who do get representation in Congress deciding that (contrary to general American precedent) their majority isn't enough.
Worth noting that, even if you do genuinely believe that a 52-48 with low-ish turnout is a good enough mandate for sweeping constitutional changes (on why this is a bad idea see: Brexit); I will never fail to remind people that PR has an anti-statehood territorial legislature.
So admitting PR as a state sets up immediately a constitutional crisis when PR's territorial legislature simply refuses to write a state constitution.
The pro-statehood governor they elected at the same time as choosing to break their chains of oppression can call a state convention to get it done, and then they will elect a new state legislature under their state constitution.
This will take time, and that's fine. Puerto Rico won't be a state in early 2021 like Douglass will be if Warnock and Ossoff win.