Working in the finance areas and doing a lot of research on credit cards, I'm told (and maybe this is just rumour but it makes sense as my credit score is considered excellent from the reports i've paid for) that while credit scores are taken into consideration, it's also behaviours of people like us who do like signing up to high points credit cards, using them and then cancelling them after we receive the points and moving onto the next promo. We pay off interest quickly, dont have any loyalty and that doesn't stick with many banks.
Everytime you apply for a credit card, it's a mark so I think it sounds like there has to be some timeframe you have to wait before acquiring another (I did three last year, cancelled two and since then I get instantly declined for any I apply for and I'm told I should stop doing it and wait for a few years before doing it again!)