Because with initial charges, when you've got pretty solid probable cause for intent (2nd degree), there's no reason to take any risk at the probable cause hearing with premeditation (1st degree); if you've got evidence that you think can get a conviction on that as you investigate, upgrading charges is easy.
It's really, really hard to get prove exactly what someone was thinking. You need a very concrete piece of evidence that proves intent, just going somewhere is not going to be enough.
How about going there completely disguised, and pretending to go to a different floor in the building than Fahim? If he went there to talk to him, he wouldn't press a button in the elevator for a different apartment.
Even weapons being brought could have been intended for threatening future violence.
It's incredibly hard to prove, beyond reasonable doubt, what someone's mindset is at any given time.