Well, it's because it's not issued by a "Trusted Certificate Authority" like the "real" ssl certificates used on the internet. (for example issued by VeriSign)
What you can do locally to work is to add the certificate manually in the Trusted Certificates for your user and/or local machine. But this procedure must be done for everyone using it until you will obtain a valid SSL certificate issued by a CA (certificate authority).
But your question points to the scenario where it's for dev purposes only so what you can do is either manually add the certificate to Trusted or you can override the certificate validation mechanism in .Net and write code that will consider your certificate valid.