Their best selling Lumia 900 is already obsolete after the announcement that it will not be upgraded to WP8. So people will not buy an obsolete device. So they have to release their next generation Lumia in order to capture the market. Lumia 900 was a hit despite having mediocre specification. But those specifications were because of OS limitation. With WP8 I don't know Nokia should feel reluctant in hurrying its next generation high end Lumia to capture the market again.

PS: I do feel a slight hope, of MeeGo's rebirth, When Nokia chairman Risto Siilasmaa about their Nokia's contegency plan in case WP8 fails