Dropping support at that particular point in time was purely a marketing / management decision that wasn't driven by any technical considerations whatsoever.
When you do this kind of thing as a company over and over again, people notice.
I think that covers most of the main points.
Of course, it may be that it all came down to support contracts internally between Google and the Nexus device suppliers (the Nexus 7 blobs come via Asus, the Nexus 4 was an LG device that used the same Qualcomm chipset IIRC.) but that’s the kind of detail that end users really don’t care about: both were Google devices that Google sold & from the point of view of the end user Google dropped support for one device for no obvious reason as far as the end user can see.
1 - Google has a history of living up to the absolute bare minimum promised. Apple does not. Per history, one can assume that when Google says they'll support for 3 years they mean 3 years and one day, and Apple means about 5.
2 - Google has a history of whining as if they are absolutely powerless, broke, and incapable of affecting what Qualcomm or others do. At least when it suits Google.