It was more along the lines of how it was written. If it was a collection of declarative statements reflecting your opinions and what facts you feel support them, there is little reason to expect a counter argument. On the other hand, if it compares specific elements of the bill to various precedents and facts, pointing out the conflicts in the bill and asking what his position on those points is, then there is a clear opening for discussion.
For example, if I say "I hate elephants", the other guy can shrug and say OK, acknowledging that I said something and nothing more. On the other hand if I say "I hate elephants because studies show they fart a lot and this contributes to geen house gasses. What is your position of farting elephants? Do you agree or gisagree and why? And if you disagree, how do you justify that position given the facts?" A simple OK just doesn't work, they either have to ignore a direct question or answer with a meaningful response.
Politically, ignoring a constituent who has taken the time to write looks bad, and a non-response response to specific questions looks even worse, an so he is stuck and has to reply.
This ability to avoid a response is one of the reasons I meet in person, there is just no way not to respond.