The story everywhere with respect to the Nexus One is shortage of supply due to lack of availability of the Samsung-sourced AMOLED screens used with the phone.
Google would almost certainly have been informed of the coming supply crunch weeks ahead of the wider market. They announced on May 14th that Nexus One availability was about to change. It was, after all, the only product sold in their online store. Google appear to have tried to reserve their remaining stock for developers as the phone is the reference platform for development of Android v3.0. Those have now sold out, too.
Looking in the rear-vision mirror, it looks like a case can be made for Google having decided to pull the plug on the online store at least in part because they knew they would soon face a supply crunch that would last months.....if it didn't last until the EOL for the phone itself.
Samsung are spending US$2.1 billion on a new screen factory that will allow them to almost double their annual capacity from 35 million units to 65 million units. Meanwhile, HTC have moved their phones to SONY's Super LCD.
Maybe we'll see a Super LCD Nexus One?