The trivial answer to this is "the piano keyboard is already a standard, everyone would have to relearn everything, etc." but I assume you're asking the question assuming 'all things being equal'.
I suspect certain types of runs and patterns are much easier to play on a one-dimensional layout. A piano player can (mostly) play without fear of also hitting a key behind, or in front of, the one they are playing. (I say mostly because there is some risk of doing it with white/black notes!).
A one dimensional keyboard also makes it easier for the player to position themselves such that they can strike the keys comfortably, repeatedly, with sufficient power. Having to do this while having to reach forward and back different amounts would presumably be more awkward.
Of course a one-dimensional keyboard isn't necessarily a piano keyboard - it would be possible to use a one-dimensional isomorphic keyboard that was simply a row of identical keys, but it would be easy to get lost on an instrument like that without the use of further visual aids. Incidentally I do expect to see more instruments appearing with increased visual feedback to help with this problem.