The 24-year-old, who played the role of Zoe Slater for five years, quit the soap in 2005 when her character's affair with Dirty Den Watts (Leslie Grantham) was exposed to the residents of Albert Square.
Speaking to thisisbristol, she said: "I just got quite bored in the end, to be honest. Doing the same thing day in, day out. It's just so boring. I like to jump from different things."
She added: "Money doesn't drive me. You couldn't get me back."
Ryan went on to star in BBC One drama Jekyll before moving to the US to take the lead role in the now-axed US remake of The Bionic Woman.
This weekend sees her return to UK screens in a four-episode stint of BBC One's fantasy drama Merlin.