Waiting is challenging, but, IMHO, it's because crime and prosecution in real life isn't like TV shows or movies that are "relatively" simple (with a limited number/amount of happenings, villians, accessories and evidence) and (need to) resolve within 30 minutes to 2.5 hours or a even few seasons.

Fani is likely a cog in one or more RICO ops. While all cases should be as tight as possible, the stakes are higher with RICO because lower-level minions like her are key to the higher-ups. Getting them with the best-formed, tightest, dead-to-rights prosecutions might mean that, strategically, they can't deal with her until they have critical-mass evidence of her RICO connections and other offenses to potentially get her to flip/rat on them. (And, in the meantime, it's possible she and her affiliates commit additional crimes.)

Reminder: It took Rudy Giuliani 10 years to roll up the New York mob.