It seems like the majors hire more from regionals than cargo?
As others have pointed out, the majority of Dispatcher positions are at the regional airlines. It follows logically that the majority of hires come from the regionals.
What has consistently struck me is the "ignorance" that mainline/major people suffer from regarding their regional counterparts. I still remember a conversation I had with a Delta Dispatcher, back when I was just starting out. He expressed genuine astonishment when I told him what I earned. He'd spent his entire Dispatch career at Delta, and had no idea how the other half lived. It logically follows that this ignorance exists at the leadership and recruiting arenas as well. If there is a bias against hiring from Supplementals, this could be part of the reason why.
There isn't a career formula for getting hired at a major. There are too many variables at work that you have no control over. If you understand that, you then realize there are a couple of things you can and should do.
The first is to evaluate any job opportunity strictly on its own merits; compensation, working conditions, duties and responsibilities, stability and outlook, location, opportunities for advancement, etc. Don't evaluate a job on what you think might come after it. That way lies madness, because things happen, and you'll drive yourself crazy with the "what ifs" and "if onlys." Play the hand you're dealt, not the hand you wish you had.
The other thing you can do is actively work to advance your knowledge and skills. I've said it before. After a couple of years in your first, entry-level position, you'd better be working to either move up to a position of greater responsibility, or move on to a position that offers the opportunity to acquire new skills.
And of course there is professional activism; networking, professional group membership, etc.