Many feel that conversion is simply not important or necessary for them. My wife and I went through a painstaking process in deciding to raise our children as Jews. I didn't want to convert, but would do what was necessary. I learned Hebrew, I joined the Brotherhood, to show my commitment and example. I think commitment through showing and doing is more important than conversion.
Many people grow into the Jewish community.
They adopt Judaism as their form of worship. People are on different points on a continuum, it's hard to pinpoint where they are. It is very difficult, if not impossible, for a rabbi or a movement or a Jewish leader to call for conversion without conveying the message that those who do not convert are less valued, less worthy, deficient in important respects. Being made to feel different and unwelcome can hardly be conducive to the efforts of these parents to raise their children as Jews. It took us a long time.