There is no money in it for the dealer to "train" a mechanic on how to work on a $2000 tractor, the real money is in the $30,000 + tractors.
Normally I'm with NIWolfman 100% but on this topic I'll have to differ. To me it *IS* fair to hold a dealer with a certification on repair of a given make to know that makes units, 40 years or 100 years old. I have two hunks of old iron I work on, Allis-Chalmers and Oliver and I could take either back to a dealership that now holds their certificates and those mechanics would be able to fix it. The older the unit is, the more likely it was designed for field repair and the easier to fix.
We are so spoiled today with trucks aplenty that can haul these heavy beasts around. Some of those older tractors never left the farms the trucks delivered them to, and that is how they were built, so the farmer could fix them.
Again, no sorcery here. Why would I trust my shiny new $100,000 high tech tractor to a mechanic that can't fix an antique with systems that were simpler? If I were a dealer, to a potential new mechanic I'd point to an old hunk of junk in the corner and say "That needs it's engine rebuilt, do it and diagnose any other running problems, you have 2 days". I'd keep changing whatever needed fixed on that hunk of junk, but in 2 days I'd know if they knew the basics and how they handled their tools, their skills, etc. Nice thing is, after going through a few new applicants, it would be a nice restored tractor I could keep or sell for $$$.
Old iron is the baseline forge that a mechanic should be aware of, even a newbie with a shiny new mechanic certification. If you can't fix that, you were asleep in mechanics school, modern stuff evolved out of the older stuff.
To modernize the idea, I am a fully certified PC technician, college educated and have been in the industry for decades. I deal with newbies all the time. They come to me with their shiny certs in hand saying "I know it all" and I point them to an old computer and say "Video is down on that machine, fix it." If they were awake in class, and have two brain cells to rub together, they can get it fixed. If they give up or aren't very bright, I know I don't want them working with me as they'd be a waste of my time and money to employ.
There's no excuse for not being able to work on an older unit. What is possible is that the older unit may not be worth the repairs needed to get it running, and that is the customers call, not the dealers.
Sorry for the wall of text, but it's kind of a sore spot for me to make excuses for poor quality technicians. I'm not saying NIWolfman is, but I set the bar high for dealerships, they are required to stay on top of the new and the old in my book, that's why they can charge more for their services than the independent mechanic, at least to me.