1- A dealer is expected to meet certain quarterly sales objectives by the manufacturer. If he doesn't, the company may drop the dealer and give the line to another dealer in the same sales area. I was a customer of one N. California dealer who lost ARC for this exact reason.
2- Sometimes, try as he might, a dealer is simply unable to interest enough customers in a given companys products to justify keeping the brand in the store. I was a customer of one S. California dealer who dropped Rowland for this reason.
3- Many manufacturers have an extensive product line, ranging in price and performance from entry-level to State-of-the-Art. Some expect or even require its dealers to carry the entire line, others don't. I was a customer of a dealer who wanted to sell the older, economy-model Vandersteens, but at the price point of the upper level models preferred to recommend and sell the Wilson Audio speakers. Understandably, this was not acceptable to Richard, and the two parted ways after having had an excellent working relationship for many years. I thought the dealer had completely blown it, but he was a stubborn guy. RIP, old friend.