1) They are a very simple design with basically one moving part. Whereas as bifters (brake/shifters) are complex and difficult to repair.

2) Shimano shift cables on bifters get in the way when using handlebar and rando style bags.

3) I think non bifter brakes and bar end shifters are cheaper than bifters.

I am wishing I had downtube shifter brazons added when I had my DeSalvo built. I would like continue to use my Ultegra bifters but the down the road flexibility would have been nice.