A lot of companies are afraid of taking TOO big of a contract from Apple.
Apple has a habit of deciding to change and/or cancel contracts suddenly (and I'm sure they have an awesome team of lawyers to make it stick) after a manufacturer has dedicated an entire manufacturing line to making the (very specific) part for Apple.
I worked for a company that made parts for Apple about 5 years ago. Apple kept telling us to knock down the price or they'd cancel the contract. Eventually we were selling to them at a loss. For some reason our spineless management kept kowtowing to Apple and we went bankrupt. First thing the new management did after bankruptcy was to tell Apple we would never manufacture for them again.
Unfortunately we got sold to another company...who just signed a contract with Apple. But the parts we are making are not Apple exclusive, so we got that going for us, which is nice.