It isn't a secret that Nokia was working on this phone for a while and their exclusive deal with Microsoft prevented them from releasing it until now. Part of the reason why MS likely acquired Nokia now was because the contract was set to expire and they could lose their largest handset manufacturer. From Nokia's POV, they've been making this for a while and why not show off the hard work? I am sure it is a bit of an ego thing on their part. And with the timing, in the event the regulatory stuff prevented the purchase from Microsoft, it is a good idea for Nokia to keep proceeding as usual and go ahead with the release.
Remember that Nokia is only selling their handset side of the business to Microsoft, with a 5 year use of the name. After that time Nokia may consider getting back into the mobile space and what a nice way to come back by having a product that may wet a few appetites (it worked with their N9 and Meego, look at the diehards for those on /.).