Because this device has much more mass-market appeal than the previous devices. The tablets released to date were about attracting developers and power users to the platform (who, by and large, aren't particularly excited by the sorts of applications general consumers tend to expect).

With this new device, Nokia is looking to attract a much larger—and, thus, mainstream—audience. With the increase in users you're going to see many more developers, in particular, you're going to see many more developers targeting that new mainstream audience.

It's really as simple as: more users equals more applications.