Gory details as to why and when
http://www.iep.gmu.e...History_Paper.pdf
If you look only at the formal 802.11 standards, it looks like a and b came out almost together. On the ground, the informal b precursors owned the market. a never gained traction because it would have forced replacement of all equipment with significantly more expensive kit.
Without collisions, bandwidth gain at the time would not have been enough to warrant a wholesale upgrade. Even wired networks were only starting to go beyond 10 Mb hubs at the time.
