John Wells, speaking at the LA wrap party for the medical drama, which is about to air its final episode in America, said there have been "lots of requests from [US network] NBC in the past to spin the show off".
He went on: "But [ER co-creator] Michael Crichton and I always felt very strongly that we shouldn't - that it was a world unto itself and it would actually weaken the central show."
This last season of ER has seen past stars including George Clooney, Julianna Margulies and Noah Wyle reprise their roles.
"Everybody wanted to do it," said John. "It wasn't that hard to actually convince them, it was just figuring out around their existing schedules. We also wanted to make sure we had the stories that would service them well."
John also revealed he thinks it was the right time to end the series, which has been running for 15 seasons.
"Those are always the most difficult decision to make," he said. "There were a few cast members who were talking about leaving at the end of this year. We lost a couple of cast members that were very central to the show last year.
"We've replaced cast members over the years, but I didn't want to try to introduce a whole new group of people for only one or two years. I felt we had done it really well, still really proud of the quality of the show, proud of the people who were in it, and it seemed like time."
The last season of ER is showing on more4 at 9pm on Thursdays.