Diesel engines why not more available in smaller vehicles ?

It's almost exclusively about emissions requirements. Europe has much more lenient rules regarding diesel emissions, so with the 40-60mpg you can get in real life they are very popular.

In the US the emissions requirements are crazy. It's not unusual for a 20yr old 1 ton to get better MPG than a modern diesel pickup.

The new diesels are night and day different from those of 30yrs ago. Quiet and good acceleration. Of course, gas engines will hold their own in terms of long term reliability.