In my experience this is about an economy of scale and lack of understanding of the market.

Straight companies quickly figure out that they have to spend similar amounts of money to create and market a gay site, and although the ROI in gay traffic is strong, the actual dollar amount when compared to the monies received via their straight sites is too low to justify the expense.
A lot of these companies are working to a very tight margin and they need to keep a certain dollar amount coming in to keep the lights on. The gay market is simply smaller, and although the conversion rate might be higher, it isn't high enough.

Lack of understanding and research is the other issue - they don't know why they are making content, they don't really understand the market or sub-niche. They also don't know where to market the product, who to talk to and how to measure the results.

I suppose I can also add a certain amount of homophobia (although this is not an "across the board" comment). I have encountered straight companies run by people who are homophobic and vocally so. They will never, ever succeed in the gay market.

Mary