It's very hard for a software vendor/author to actually gauge how good their products are unless they charge money for them. Free software distorts the normal feedback mechanism of the marketplace, which is getting someone to part with their hard-earned money. So, people use free software products (not all, but many) primarily because they're "good enough", given that they are free. This leads the software vendor/author to assume that their existing users will gladly pay money for the software, but sadly, this is often not the case. Once you start to charge money for something, "good enough" is no longer "good enough", and the conversion rate is very low.