Why many of DELL laptops still ship with Windows 7?

Hello,

I am thinking of purchasing a DELL Laptop, probably the Inspiron series. 

I see that many of DELL laptops still come with Windows 7 and this is even a filter criteria when browsing laptops on DELL.com

There is "Windows 7" and "Windows 8" filters.

Is DELL not confident in Windows 8 or what is the reason ?

Also, if I purchase a laptop today, would you advise me to choose Windows 7 over Windows 8 and reasoning why ?

Thank  you.