I say it's because "follow the money."

Texas Tribune article:
"It’s cheap, easy to make and in demand overseas. So why can’t this Texas-born COVID-19 vaccine break into the U.S. market?"

Excerpt:
Now, nearly 18 months later, the Houston team’s vaccine, called Corbevax by its maker in India, is cheap, has no patent, can be made by many vaccine producers globally — including those in low- and middle-income countries — and is poised to receive approval for widespread global use.
The Indian government has promised the biopharmaceutical company Biological E Limited, which is making the vaccine in that country, that it will buy 300 million doses with the potential for more.
A halal version of the vaccine, for use in Islamic countries because it doesn’t contain animal-based ingredients, is also about to start clinical trials in Indonesia.
And later this year, the company hopes the vaccine will be endorsed by the World Health Organization for use globally, which could open the doors to quicker authorization in several countries that need it.

Link:
[click2houston.com]