But what makes you say the Kovacs novels are "more popular"? You prefer them? A group of your friends prefer them? You know some people who prefer them?
Black Man was clearly a very successful book - although not enough for Morgan to "retire on the proceeds". Incredibly few sf authors earn enough to do that - in fact, I can't think of any (still alive, that is). Fantasy authors, OTOH...
I still like to know what prompts you to make that claim about the Kovacs novels.
What prompts me to make these claims about the Kovacs novels is the numerous comments that fans of the Altered Carbon novels have made themselves.
There's hundreds if not thousands of them to read on the internet, knock yourself out.
You could make a start with the reviews for Black Man at amazon.co.uk, as that's an easy search.
Then start checking out SF ezine/book/magazine reviews and SF blog sites and then individual fans of his Altered Carbon novels.
Sure many, many readers enjoyed Black Man and his other books, but a common thread that runs through a lot of the reviews plainly says that they (the person writing the review) preferred Altered Carbon and the other Kovacs books to anything else Richard Morgan has written. Including Black Man.
As I have read a couple of hundred reviews of Richard Morgan's books in the last couple of years I feel that the evidence speaks for itself.
Others might choose to view the evidence in a different way or not believe it at all.
That's up to the individual.
The Internet is a big place and Google is your friend, the truth is out there.