Probably because this week had some other fairly decent stuff on, like Criminal Justice on BBC1, as other weeks have had ok stuff on. It doesn't help when ITV don't announce anything. I don't watch ITV so there was no way I was going to know when the series came back on. If I didn't search online all the time, I'd never have known it was back and would have missed the entire season. At least BBC always announce stuff, even if it is just "coming soon" on the web. ITV had nothing on their The Fixer page until four days beforehand.
They need to step up to the plate.