Torvy, I can't speak to tractor dealerships and their manufacturer connection, but I can sure speak to dealership/manufacturer relationships in other businesses.
Today, in any market, every good equipment dealership has a tractor line - the good lines. John Deere, Kubota, Case. So if a manufacturer new to the market is looking for a dealership, it has to choose from someone who wants to get into the business (remember 95% of new businesses fail within 5 years), or try to convince a "Kubota" dealership (for example) to take on it's line. What the late arrival manufacturers have is the runts of the litter from which to choose.
Then there is the "salesman" issue. Many have written of issues with no call backs, no knowledge, not caring. If you're a good salesman, you want to associate with a dealership that has inventory, good service, and will be around for years - again, look at the name brands. If a man's looking for a job, a few square feet of floor to stand on, selling an off brand brings in the pittance each week - which may be all the "salesman" is worth.
I much prefer to buy used from the owner - then I don't have to deal with fools that have reached their level of incompetence.