Because Harmattan is essentially an API-compatible version of Maemo. Maemo, as a partially open platform, had a limited future.

Maintaining an entire OS like Maemo was out of scope for Nokia, and it was in their best long-term interest to move the OS itself outside of the company, where lots of development work was happening already. They got Intel in on it, and over time it would have let them contribute to dedicated teams on the core, while reserving their in-house teams for porting work and Nokia-specific customizations (something they desperately needed to focus on.)

In the meantime, they had the inevitable issue of transitioning while not getting stuck, so Harmattan exists between Maemo and MeeGo, with the underpinnings of one and the APIs of the other. All the while, being stymied and stalled by an uncooperative bureaucracy.

I think "forced" is a bit silly. Well, "forced" by a light breeze into adopting the OS of his former employer...

I don't believe, not for a moment, that Maemo/MeeGo are in any way at fault for Nokia's inability to release a device based on them for the better part of 1.5 years. That screams "internal political failures" much louder than software related issues.