The 'Beautiful World' singers, who announced their split last month, defected from Cowell's Syco to rivals Sony RCA in March.
Speaking to The Mirror, the boyband argued that Cowell had become more concerned with his own celebrity status than their career.
"Leaving Syco was us trying to patch up cracks. We felt unloved because we were making albums with that particular label and we were having to make our own decisions," Nicky Byrne commented.
He added: "Simon was the best thing that ever happened to us, with Louis Walsh, in the beginning. Without Simon Cowell we wouldn't have had the career the way we've had it. He picked every single and it's the singles that make you successful. He's a very talented man at what he does.
"But then Syco Music... that whole machine exploded for him in the States. I don't think for one minute, 'Why is he gonna worry about little old Westlife when he's over in the States?'"
The band also claimed to have agreed that the release of their Greatest Hits album would be the "natural" time for them to split.
"It kinda felt like now was the time," Shane Filan said. "It was a conversation we didn't like admitting. But we all did admit it."
Westlife will release their Greatest Hits album on November 21, before embarking on a farewell tour next year.
> Westlife: 'Simon Cowell hasn't been in touch about split'
Watch Westlife in the video for 'Lighthouse' below: