So you still did not answer my question as to why the Torah forbids a Kohain from marrying a convert.??
Actually, there is no such mitzvah against a Kohein marrying a convert.
There is a mitzvah against a Kohein marrying a Zonah — and all converts (as well as some non-converts) have the status of a Zonah — but there is no specific mitzvah prohibiting a Kohein from marrying specifically a convert.
The Wolf