Clives point is spot on, the Deere is such a strong name that contract hire companies are never going to risk their margins by hiring out Case or New Holland, no matter how good the product. The bottom line is profit, and JD tractors do not lose anything like the amount of money other tractors do. Yes, you maybe pay more in the beginning, but it still works out better in the long term to buy Deere.
Take a trip to Cambridge sales and look at the prices that different tractors fetch. It is no accident that JD fetch higher residuals, they are a better machine. An ex-hire JD6910 will fetch a good price regardless, but an ex hire case MX135 will struggle. The market dictates the patterns, if hire companies will only supply Deere then there is a good reason, whether you agree with it or not.
By the way, I run Case and MF tractors and have always bought second hand, so I am not biased in favour of JD. The MF is the most economical tractor in my case (very cheap to buy second hand, cheap parts, reliable engines) but for hire fleet JD are kings and rightly so.