This decision was likely motivated by two factors.
One, it respected the Islamic tradition of burying a deceased individual within 24 hours. Two, the US didn’t want his grave to be a shrine to Al-Qaeda members and sympathizers. Therefore, instead of burying it in a location on land, they buried him at sea.
Even after Bin Laden’s death, the US was still wary of the threat he posed the Western world.
He will undoubtedly be painted as a martyr by Al-Qaeda and its sympathizers. If he were buried on land, his grave will likely become a prime recruitment site for Al-Qaeda and suicide attacks.
If he wasn’t buried according to Islamic practice, Islamic extremists would have more fuel to accuse the US of disrespecting Islam and waging a religious war against it.