Why are no recent small aircraft designed to be "characteristically incapable of spinning" as the Ercoupe was?
The coupling between the ailerons and rudder are designed such that they avoid uncoordinated turns as you described.  However, this occurred at the expense of control in other flight conditions. 

In fact, there are situations where you need to deflect the rudder without roll input, you need to deflect the rudder more than the preset coupling, or in a manner that is opposite to the preset configuration. 

For example, if you turn right, the system will add right rudder to keep the airplane coordinated. But if you want to do a forward slip, you may perform a left hand roll, but with rudder all the way to the right. The system would be incapable of performing this maneuver, meaning you can't fly in high winds with the system installed. This is fine if you live in an imaginary place where the winds are always favourable. 

As a result, a common modification on the Ercoupe is to disconnect the coupling between the rudder and ailerons... 

Edit: I also wanted to add that spins are recoverable, within reason, by a trained pilot. For some planes, you are simply forbidden to intentionally enter a spin as a remedy to its inability to recover from a spin!