I've run the endorsing artist program for several companies, and have been an endorsing artist myself for about a dozen companies.
It really gets down to this: what can you do for the company to increase sales. Nothing else matters.
Generally, manufacturers are only interested in artists with a national or international following. They look for artists who are visible in the media. They are generally NOT interested in artists who only have a local or regional following and who distribute their recordings themselves. An artist seeking an endorsement arrangement needs to bring something significant (other than playing ability) to the table.
Every company I know of gets lots of requests every day from people who wish to endorse their products and, in turn, recieve free product or be featured in advertising. The decision always boils down to " what can you do for me that the other applicants can't do?"