People had been using this system for over 20 years. Muscle memory from hitting Tab, Tab, F2, enter...that pulls up the inventory detail. F5, 3, 6, Tab, that brings up the wholesale price.
Now, with the 'better' system, people have to move the mouse up to Lists, Items, wait for the Item page to load, then choose the item the want, wait for that to load, then click on the Inventory or Pricing tab, wait for that to load. They can save a few steps by typing 'item: 25630' in the search bar. But of course it loads about 5x slower.
You could also hit PgUp/PgDn to cycle through items. Now you have to click the little arrows at the top of the page, and the view resets each time, so if you were on the Pricing tab, you have to click on that again. The old system left you where you were.
Sure, the new system has all kinds of advantages in other places, but to most of the internal users, you just ruined their world and they hate the unfamiliar, slower system that costs millions of dollars.