The choice of fixity came about because actually the most common thing replaced by (<&>) is actually (>>=), when the thing you are binding to no longer has an effect, not actually (<$>), despite what the name suggests.
This made a non-trivial difference in the amount of parentheses in real code, and was a conscious decision, so reverting it is not something I would do lightly and breaks real code.
Back during the discussion of whether we should adopt (&), (<&>) also came up, but with only one voice in favor, and nobody else really feeling passionately, and with various colors like this available for the bikeshed it was dropped.