A.V.I.A.F.
Captain
They probably didn't want the inadvertant humor of his falling on his ass to make the episode get off to a bad start.
Well, they could have done it without it being too slap-sticky. That scene does have a degree of levity to it anyway (i.e., Christopher looking totally confused while Kirk smiles smugly).
Although he does beam aboard with his back to the transporter control console. The only other times I can remember that happening are in Enemy Within (which I guess was done to convey the whole alter-ego concept), and Day of the Dove (when the Klingons are beamed aboard).