The combative midfielder had held talks with Russell Slade, with the Town boss seemingly in pole position to sign his man.
However the lure of staying in the South was too much for Simpson who opted to sign for the O's on a two-year contract.
"I had a chat with Russell Slade a couple of weeks back but I wanted to come and see what Martin Ling had to say," he told the official Orient website.
Town miss out on Simpson
"I was very impressed and now I’m here and I’ve signed a two-year contract. I don’t know what happened with Bristol Rovers but I was impressed with what was on offer here and I’m happy to be here."
Slade must now turn his attentions elsewhere for a midfielder to presumably partner Stacy Coldicott next season.
Slade must now turn his attentions elsewhere for a midfielder to presumably partner Stacy Coldicott next season.