Mainly because the plumbers don't like the Waste King.
We do like Insinkerator. Insinkerator has a nice "in home warranty", they will come to your home to fix or replace.
You can buy the cords, air switches, and replacement sink flanges at a hardware store if needed.
If something falls in, I can put my hand in there to pull it out, I can't do that with Waste King.
They are quieter. Some of the Waste Kings are Gawd Awful Loud! I had some 3/4 hp Waste Kings that sounded like a jet taking off.
Do not, I repeat do not buy a Costco Titan disposer. Super lightweight plastic body, it feels like you could play touch football with it. The inlet for the dishwasher is too small, and the water will back up the air gap and flood the kitchen countertop. They must buy those things for a few dollars in large containers.
I used to do work for one of the Costco "buyers" in the Seattle area. I quit doing work for him when he wanted my to plumb a downstairs bathroom into the rain drains. Just below him was Lake Washington. I swim there.
The Insinkerator Evolution series is killer!
Super quiet, you can see the video at this link.
Super Quiet Evolution Link
I like how it shows celery being cut up. I can remember snaking a kitchen drain for a family that ate a lot of stir fried food.
It was pretty clogged up with stringy celery. I've seen some disposers that do a poor job on carrots and potatoes too.
Seattle area disposer installation