As always...follow the money.
Actually, schools get money for any special ed classification, so mental retardation (or as the psychiatrists now tell us to call it “intellectual disability”) and autism both get them money.
The only place I see a skewing in terms of financial incentives is in the way private insurers like BC/BS treat them: evaluation for intellectual disability is usually not covered, while evaluation for autism is. (The bean-counters think the schools should diagnose mental retardation, but think autism is a “medical” problem, even thought the psychiatrists list both as disorders in the DSM-V.)