Nigeria is a very brutal and corrupt nation so any military assistance given to it could easily be diverted or even sold or outright given to Boko Haram. Without substantial assurances that this would not occur, it would be too dangerous for the United States to provide Nigeria with weapons technology.
Also, as Quadri Oluwaseun noted, no nation has a monopoly on weapon’s sales. Nigeria can purchase weapons as necessary on the open market or from other nations. That it did not, especially when its citizens are being kidnapped and murdered is very telling.