Nest could be waiting to see how Lockitron fails. Then, they would have the chance to compete and innovate on a better product. Given their current mainstream success and apparent financial comfort, I would imagine Nest having more resources to tackle the problem than Lockitron can hope to raise with a crowdfunding campaign at this point, in addition to more years of industry experience. So that might explain why, even though Nest could make a door lock right now, they are waiting for the right time to develop and unveil one.

and what exactly does lockitron have to offer Nest?

Turn-key addition to their core product offering.