[DaveNP]

I can think of two scenarios to explain this
1) Cost effectiveness (aka - making bonus payments). In a a modern garage dealing with modern cars the usual way to work on a car is to plug it in and read the codes then replace the faulty part, it's reasonably quick and you make profit on the sale of the part, if you've got an old car you've got to start by using a process of elimination to get to the fault, that's time consuming, time that the customer won't want to be paying for at £100+ per hour so the workshop manager will be on your back.
2) The variability of people. As already suggested by Familyman some of them would love to get under the bonnet of a classic, the young mechanic I know had an old Porsche to work on in his mum and dad's garage before he had left school, you've just got to find the right lad. On the flip side I know a very good old mechanic who can take a short ride in my car (with me driving) and tell me what will need fixing soon, but give him anything with electronics and he just shuts down