Google didn't expect the Nexus 6 to be in high demand, hence the supply constraints
Of course, all this means that there's demand for the Nexus 6. Google today reported its Q4 2014 results, and, during the earnings call, the company's CFO, Patrick Pichette, also talked about the Nexus 6. Pichette said that the device was "very well received as a new phone," but "we had real issues and were unable to secure sufficient inventory to meet the demand we had forecasted."
Unfortunately, Google's CFO didn't elaborate on what forecasted demand meant. Thus, we've no idea exactly how many Nexus 6 units were sold until now. In any case, given its high price (starting at $649), it's somehow understandable why Google didn't foresee that it wouldn't be able to meet demand.
Besides Google Play and Motorola, other places where you could find the Nexus 6 in the US are Best Buy, AT&T, Sprint, T-Mobile, and US Cellular. Have you bought one yet?