Pietersen's central contract was cancelled shortly after the whitewash defeat to Australia in the winter, with some figures in the game criticising the ECB's decision to cut ties with the influential batsman.
However, Downton suggested that there was no support from the dressing room for the 33-year-old before the announcement was made that he would no longer be available for selection.
"I was quite frustrated watching him as a fan, and there was a feeling he wasn't engaged as he should be as a senior player. I then watched every ball of the Sydney Test live, and I've never seen anyone as disinterested or distracted as Kevin. It looked very strange," Downton told BBC Radio 5 live.
"I talked to every person on the management team and quite a few senior players, and I couldn't find one supporter who wanted Kevin to stay in the side. We spent the next three weeks working out what to do in the best interests of English cricket.
"Kevin was starting to play a bit like a luxury player. There was a 'this is the way I play' type of attitude. He had not been fit an awful lot and he wasn't producing the results he once had."
England continued life without Pietersen by beating Sri Lanka in the first one-day international at The Oval on Thursday.