It's one thing to handle different parts from the same supplier it's another to switch suppliers completely. Completely messes up supply lines, part stocks, procurement handling, logistics, tooling, etc. We're talking about hardware here, it's not a relink away.
IIRC (and please correct if you have any proof to the contrary) it went like this:
1. New CEO decides to partner with Microsoft. Winphone requirements demand massive hardware reorientation
2. Nokia can't produce winphones at once. To held self-imposed deadlines Lumias needs to be subcontracted to one of the usual taiwanese winphone producers. It wraps an N9-derived shell around something very close to the Microsoft reference design
> Nokia outsourced the production of its Qualcomm-based Lumia 800 to
> Compal Electronics. According to Nokia, this was due to time constraints
> and Compal's experience with the chipset.
What madness is that? You don't externalize your core business ever. That's a basic rule. And if Lumia production was not Nokia's core business at this point why did it kill everything else?
3. Nokia publicly disparages its old product lines, tries to strong-arm all its parters (carriers, resellers) to accept winphones instead of what was previously agreed. Refuses to sell other lines if the partner doesn't push Lumias
4. Everyone switches to Android, Symbian tanks, Nokia does not want to sell Meego
5. Consumers recognize the taiwanese winphones they didn't want last year under the new Nokia cover and don't buy them
6. Nokia closes factories
I don't know if between 2. and 6. Nokia made any serious effort to re-internalise production, and even if they did (and taking into account that Compal almost certainly insisted on some volume commitments before producing) Lumia sales are so underwhelming they can't justify retooling most existing factories nor retraining the workforce.