I can think of a few in-universe reasons:
First, it's possible that the presence of the nanites would have interfered with the targeting system on the ICBMs (see below). Randall could, of course, have just calculated the correct ballistics, launched them, and crossed his fingers, but given his intention to off himself, he wouldn't get a second change if he screwed up. With the electricity back on, the computers at The Tower can control the trajectory of the ICBM's for their entire flight.
Second, its possible that the USB controllers are too rare and he didn't want to waste them without knowing they would work, or possibly he didn't even have enough of them to fit onto the missiles.
Third, it may have all been part of the plan to get the power back on before launching all-out war against the various nations. This way, the "real US" now has their power back on and can use it to swoop in and conquer the nations that Randall just destroyed.
Lastly, he may just be psychotic and enjoyed getting everyone's hopes up before he went.
Regarding option #1, let me just express my dismay at how haphazardly the whole explanation of the nanites has been handled. Many of the things we've seen them do make no sense given how they were explained (my favorite example: how did turning off the nanites automatically start up a gas powered generator without any initial energy input?) So there's no real way to know what the nanites are capable of, and thus why Randall felt the need to shut them off before he did anything.