If I can't think of an explanation I may just fail to respond. This happens to me a lot as I get busy and forget to pay sufficient attention to social things for too long. Then by the time I get back into a social mode, there may be a whole pile of messages from different people I should deal with, and a process of triage begins where I start by responding to the ones I consider most urgent at that moment. Unfortunately some messages will be low on the list, and "why aren't you responding to me" would be especially low if I cannot come up with a good answer myself. "I am an inconsistent communicator" doesn't go down very well when deployed for the nth time!

fyi when thinking about this I am comparing my imagined p(responding) for "Why aren't you replying to me" to p(responding) for other hypothetical messages also in my "should reply" box like "Are you interested in trying out a new restaurant", "can you suggest good places for us to visit when we come to see you next month", etc. Some of those latter messages may be sent with the same intent as the first, but they are more likely to elicit a response from me as they don't require me to deal outright with the motivation behind my own social decisions.