Brand loyalty is a very strong binder. Once a photographer has more than one lens and possibly one or two other accessories then for the majority they stay with that manufacturer. Many times you will see something from another maker that is technically better or offers you that perfect feature that you wished your own brand would introduce on their next model, but when it doesn't happen few will actually jump ship and go over to the competition, simply because the cost (including likely loss on selling your own equipment) and hassle are just to restrictive on the whole.
Then there is always the issue of this year it may be your make that offers the best match for you but come next year it could be the competition. It does swing from make to make and year to year. Try to keep following that with your own equipment soon becomes a futile exercise.
Just my 2p's worth
