After the reveal, @sbeggs (who is in aerospace) and I had the same thought: why not use titanium for the exoskeleton? It's much stronger and lighter than steel, highly corrosion resistant, eco-friendly, and attractive. One of the main drawbacks of titanium - difficulty of stamping into curved shapes - has been eliminated by the origami design. It could have looked quite striking unpainted. Think of the Bilbao Museum, for example.
A quick google search on the topic turned up an interview with Elon from over a year ago in which he said of the pickup, "it's gonna have a lot of titanium." However, Ti was not mentioned in the reveal.
Here is what I think happened. The genesis of the design was partly based on the desire to use titanium for its lightweight strength. That suggested the use of flat polyhedrons for the entire exoskeleton. When they wanted the price of truck to start at less than $40k, staff probably convinced Elon to switch to stainless steel to hit the price point.
Stainless steel is the obvious lower cost alternate, but would not be a simple task to work out all the engineering late in the game - especially the appearance. DeLorean managed to make it look good, but they weren't working with 30x ballistics grade SS as the structural load bearing material. It can discolor with welding, for example. I'm guessing that they made this switch pretty close to the reveal and were still experimenting with alloys, fasteners, annealing processes, etc. That's why the truck looked the way it did at the reveal, with color mismatched body panels and a blotchy finish. The promo pix that show a lighter, more uniform silver color might be based on a titanium prototype.
I think Tesla will just need a little more time to get the skin right, along with the glass. The final product should look a lot better. Finish will be uniform, and steel frees them to tweak the design if they want. FWIW, I would have paid a pretty decent premium to get an actual titanium truck... and with all the reservations coming in on the higher end of the price spectrum... might they reconsider?