Why so few medium pace bowlers?

Test match and other international bowlers nowadays are essentially fast or slow - that is they bowl a stock delivery at about 50-55 mph or 80-90 mph (i.e. fast-medium to fast).

Why are there so few (none at all?) medium paced frontline international quality bowlers with a stock delivery of 65-75 mph?

The reason is perhaps that in spinning the ball, bowlers put at least 15 mph of 'work' on to the ball in imparting enery to spin it. In other word, they reduce the pace by about 15 mph and instead put that energy into rotating the ball.

This can be seen from observing that a spinners 'quick' (non-spinning) delivery is typically about 65-70 mph (I suspect that spin bowlers with a fast delivery of 75 mph or more are likely to be throwing); while a right handed fast bowlers spinning 'off-cutter' is typically about 65-75 mph.

The faster a bowler's 'arm' the better for a bowler, since it is mostly the arm speed which imparts energy.

So it can be seen that a bowler with a fast arm will naturally tend to bowl fast - and it is natural and more straightforward that a bowler who can propel the ball at 80 plus mph will do so as a stock delivery, rather than bowling cutters at 70 mph.

And if a bowler is naturally a 70 mph medium pacer, and given that (at first class level a 70 mph non-spinning delivery is innocuous) it makes sense to impart as much spin to the ball as possible - turning oneself into a 55 mph spinner.

So, it can be seen that a top-notch medium pacer would need to be someone who has a fast arm and would naturally be a fast-medium or fast bowler deliberately choosing to reduce their pace through the air in order to impart spin.

I suggest that this was very likely to have applied to 'the greatest' British bowler of all time: SF Barnes - whose stock delivery seems to have been a medium paced leg-spinner.

I would guess that Barnes was naturally a right arm fast-medium bowler with a fast arm, but that he was able (and chose) to bowl a finger-flick delivery as stock - using his ring finger to flick the ball anticlockwise from the front of his hand, with a normal off cutter as his main variation.

If there are ever to be international front line medium pacers again, I would predict that they will come from quickies, with a quick arm, deliberately sacrificing some of their speed to generate spin. Like Barnes they would therefore be fast seam or swing bowlers in their simple basic action, but in imparting rotation to the ball would end-up as medium-paced front-of-hand spinners.