What responsibility does Google have to spend time and money on infrastructure on products that are used by the minority of people?
Google's time and money is what it probably comes down to. Somewhere deep in the accounting department they figure out that the quality of data they are mining is not enough to offset the actual cost of running the service. It's true that a larger user base would create a larger data set and therefore be more likely to be profitable for them, but if it's just a different representation of the same data there really isn't any point. They can figure out what we're interested in from Search, what we actually buy from the email receipts in our Gmail boxes, etc. A new service like Keep might give them new information, but if not there's no reason to continue it.