A technical answer:

Voice assisted turn-by-turn navigation was already implemented in Symbian. What has changed is the licensing: paid service before and now free. This is a complex business changes followed by a rather simple technical implementation.

The same feature was not implemented in Maemo and a change in the business strategy doesn't change the fact that the feature needs to be implemented and, well, it's not a trivial one.

The Maemo 5 Ovi Maps app is not based in any of the official frameworks of Maemo 6. Therefore (again, form a technical point of view) it makes sense to focus on the Harmattan version. Let me insist that nobody has said anything about that version running or not on the N900.

Voice assisted turn-by-turn navigation for Maemo 5 will be available as a paid app from a software vendor. This matches the usual offering, even if (true) it doesn't yet match the very high standard Nokia has just set (currently for Symbian devices).