I actually couldn't get an answer when I was inquiring about me over DM, but Tay's DM response behavior seemed to go up and down throughout the day. (It'd tell people on public tweets to DM her, but never respond to DMs for hours at a time.)

This was very clearly an experiment, and I don't think they wanted to pre-train it too much, to see what would happen. I think the results were kinda predictable when people like 4chan get involved! But at almost 100,000 tweets it generated, clearly they got a lot of data to work with for the next version.