Probably because your dishwasher wasn't engineered to work with powder detergent. I bought a Bosch dishwasher a few years ago and I read the manual where it specifically stated that even though it supports powder and liquid detergents it was designed to work with dishwasher tabs so that's what they recommend.

I can't help but wonder if people buy a new dishwasher--made for a completely different kind of cleaning substance--then continue their old (powder) ways and wonder why it doesn't work as well as the old one.

Note: My Bosch dishwasher's manual had notes in it about powder like, "if you're going to use powder don't use the 'Auto' mode..." and instead you should follow their instructions in regards to things like pre-rinse and how long you should wait before running the dishwasher (like, with powder I think the goal was to make sure everything stays wet before starting but I forget).