That's reasonable. But, they made the switch without adding the security part. If you are going to the trouble to redo the infrastructure of credit card processing, why not, I dunno, make it more secure while you do it? It's not like entering a PIN number is a foreign concept to people.
Honestly, because people hate change. It's going to be easier to force one change on people than two. I don't know what other reasons were involved, but that can be a big one. We want things to just work, and work like they always have. For most people, credit card fraud is someone else's problem. People only want security as long as it doesn't inconvenience them.
Quark! Quark! Beware the quantum duck!