it's less about promotion and more about money
You make very little from being in a group
Using Cheryl Cole as an example (only because I have her figures to hand), she "only" makes £1.5m a year from GA. Now that's a lot, of course, but GA are the biggest girl group in the UK at the moment and have been around 7+ years now with their latest album going double platinum (selling). Yet for her calendar, i.e. 12 pictures, she got £500k. She got another £500k for her L'oreal deal which involved a 30 second advert and photoshoot. She received another £400k for other modelling work.
So that's £1.4m for a couple of photoshoots and a 30 second tv advert vs. £1.5m for a years work rehearsing, dancing, singing, recording, promoting singles, interviews, etc. Relatively speaking the money she gets from GA isn't that much compared to the time and effort she puts in. Her X Factor contract is about £2.5m for example.
Now onto The Saturdays, they are only in their second year and their latest album has just scraped 60k of sales since October. I wouldn't be all too surprised if they haven't got anything from their record company at the moment. Hence, they're doing what they can to earn money