The US performer appeared as a guest judge at try outs in Glasgow, and had been invited to join Gary Barlow, Tulisa and Louis Walsh on the panel full time.
But Anastacia had to turn down the opportunity, blaming her "crazy busy" schedule for the rest of the year.
"In a business that's so judgmental anyway, it was very hard to reject people - that's probably the hardest thing but you have to be honest," Anastacia told London's Evening Standard. " You don't want to destroy their dreams, but some of them really should have another dream."
Speaking about her time as a guest judge for the auditions in Scotland last month, the 43-year-old went on: "I had a great time and I would gladly help them out again, but my schedule was crazy busy. I knew going into it, it was definitely something I couldn't do full-time, but it was something they really wanted."
[tag]Nicole Scherzinger[/tag] was last month announced as the show's new judge, after also guest judging during this year's auditions.
"I love Nicole and I think she is just fabulous as a person, she's such a genuine person and I think she's going to be a really great judge again," Anastacia added.
[cat c="the x factor"]The X Factor 2012[/cat] airs on ITV1 later this year.