IMO it's their initial foray into high volume online sales. Up until this point Google Play, formerly the Google App Store, has needed to respond to a background hum of online purchasers. The Nexus 4 was to Google Play Store as Springsteen (or plug in appropriately popular band) was to Ticketmaster.
I've been in Enterprise IT for well over 30 years. Depending upon the line of business building an infrastructure that adequately responds to the median transaction rate can be entirely acceptable - so long as the company understands the customer expectations AND the Peak vs Avg tran-rate isn't on some order of magnitude level of difference. In this case it appears, again IMO, that Google was unprepared for the super-peak of the attempted transaction volume. Consumers download Apps and the other "background" noise of average transaction volume on Google Play - I'm guessing - is normally a small fraction of the tran-rate that something like the new Nexus products generated when they went on sale.
The best of breed, that know they will have super-peak days, build an infrastructure that can sustain a tran-rate that exceeds their expected peak and is typically many multiples of their normal rate. Stock exchanges, credit card companies, etc. all build this out - not out of a need to placate consumers but for real bottom line costs. For most CC processors if they aren't able to respond to a tran request within a specific amount of time (typically single digit seconds) then the request is either flat-out denied (pissed off consumer) or, more typically, redirected to an alternate processor that gets the retailer's fee and may even allow the retailer some rebate from their primary processor.
Now clearly nobody is going to miss a flight, ruin their stock portfolio, or otherwise upset their lives because Play was unable to adequately respond to their click request to buy a device. In the long run, though, when there are other online retailers that are able to satisfy consumer's needs DURING high volume request periods then if Goog doesn't get it's dung together they will continue to face challenges both from their investors and their potential online retail customers. I'm pretty sure they can fix it but admit I'm shocked that on N4 2.0 they screwed the pooch as badly as they did on original launch.