Biotic entities are conservative.  They tend to latch onto something, such as Dopamine-as-hormone, and since the stuff is floating around - note the technical terminology!  ;-) - it gets used for other things.  Dopamine in its homeostasic role had to precede its use as a neuro-transmitter -- since it's hard for a species to evolve a Central Nervous System if all the individual members of that species are dead.  

I note the thalamus has been around far longer than higher brain functioning.

Thus there may have been co-evolution going on with D-as-H (body side) promoting D-as-NT (brain side) being used by the thalamus - the lazy sod - to keep things simple.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre