Tarique,
I know of one artist who was masterful and successful as a portrait photographer and has also become a success as a portrait painter. Though this artist makes no secret of her portrait photographer background, like the sites you've seen, her site doesn't try to market the two together. I assume she's still doing both or at least, did for a while.
You do pose an interesting question, one that makes me wonder at what point and how I learned that you wouldn't try to market these two together. Perhaps it's just something that happened through osmosis over the years.
The first thought that comes to mind is that the associations people have in their minds with each area of expertise are totally different.
Painted portraiture has a tradition of thousands of years and I think in the minds of the public, it has a mysterious and esoteric quality to it. Afterall, most people could not paint a portrait that looked even decent if their life depended on it. If you told them to go out to an art supply store and buy all the materials for painting a portrait and they'd have some idea what to buy, but wouldn't have any idea what to do with the materials afterwards. And, take those same people and put them through the most intensive painting classes and a great deal of them still would never be able to paint a decent portrait.
With portrait photography, this is a "modern" invention with a tradition of less than 100 years and most people own a camera...thus, I would say that some of the mystery is lacking as compared to painted portraiture. Most people can take a photograph of another person even if it doesn't reach the level of being art and there is no special knowledge needed to go out and buy a camera and a roll of film. Even though this person would still be far away from creating a master portrait photograph, the greater public knowledge of the tools of the trade is another factor which takes away from the mystery and awe.
So, in my opinion, the public to which you would be marketing would often be entirely different types of people. I can even imagine that a wealthy socialite would love to brag about her portrait done by some big-name portrait painter, but I have a hard time imagining her bragging about a portrait photograph she'd just had done. Perhaps my experience in this is limited, but that's how I see it.
A theory...I think if a portrait painter is very successful, they wouldn't have time to also be a portrait photographer. So, there might be some suggestion that the artist isn't fully successful if they have time to do both.
I realize that some very successful portrait photographers get paid as much as portrait painters. But, I think this is very rare and, in general, the public perceives a painted portrait as more valuable than a photograph.
There might also be the danger of devaluing both by seeming to imply "you don't want to buy this, ok, I have something else I can sell you".
All that said, I wouldn't rule out that someone could come along who could do and market both together successfully.