The reason that the sale is not available on mobile devices is because the site actually has two different booking engines and the desktop booking engine is the only one that supports the promotional offers. This is also the case of the Virgin Red 20% discounts.
Depending on your device you will either be sent to the desktop booking engine or the mobile booking engine. All phones should be sent to the mobile booking engine, but results vary for tablets.
Although the desktop site is very difficult to use on a mobile device it is doable at a push and if you want to take advantage of the sale is your only option if you don't have access to a desktop or laptop.
If you're on a tablet check if the regular page is working first.
So if you're on a phone or tablet that shows the mobile version of the site you can use this link to bypass the browser checks and take you to the desktop site.