Or the equipment is otherwise expensive up-front to a small clinic. Ophthalmology, for example. The subscription feature is cheaper than becoming a financier.
Even if the TCO for the clinic were exactly the same over X number of years, the manufacturer that sells subscriptions still comes out ahead because they're effectively in the financing business, too. Why tell a customer to go borrow money from somebody else when you can make them borrow from you?
There are two reasons to use subscription pricing:
1) Price discrimination: You want to charge a large customer more for the same device
2) Lowering the capital expenditure by hiding the true price in operating costs
You could argue that (1) is good for small clinics, because they get access to the same machines as big clinics for a smaller cost. But I believe it's just a way to remove opportunities for competitors and results in everyone paying higher prices, even smaller clinics.