For the same reason I am uncomfortable with donations: It creates a feeling in some people that I need to meet their expectations because they gave me money (even if they are only giving me money as a result of voting in a popularity contest that would give me money). It allows people to say, "But you can't do that thing I don't like! I donated money to you!"

By generating all of my revenue via merchandise, I can make the argument that all money given was for the actual product--the physical book--rather than some nebulous promise to continue producing the online strip in a certain way. I prefer the freedom to alter or even (should it become necessary) cease any aspect of this website without having directly accepted money given to encourage its continued existence.

And now that the question is answered, this thread is very much done. Locked before the inevitable counter-arguments trying to convince me to accept money.