Even assuming Splashtop Streamer performance compares well to Steam's, which is doubtful considering it is not specifically designed for gaming (and considering under it's "High Performance" info section it says "
up to 30 frames per second"); there is still the factor of convenience. If a person is wanting a Steam key, chances are they already use Steam, have Steam already installed on the necessary computers, probably already have running in the background keeping all their other games up to date... and Elite already supports Steam. Why go through all that extra trouble when the only thing that is needed is a key, that I think has been pretty well established likely costs Frontier nothing.
Oh, there is also this
Yeah, that is just what everyone is asking for, yet another account to to deal with.
Edit: Also, a quick search yield
this; pretty much confirming everything I was guessing.
That SplashTop doesn't perform as well, taking more resources on the host for poorer results.
It often requires jumping through more hoops than using Steam IHS with non-Steam games.
But it is better for standard "remote desktop" functionality; which makes sense, since the two are designed for different purposes.