Why direct injection gasoline engines don't use 20:1 compression ratios like Diesels?
 

So they can combine gasoline clean burning and widespread availability with Diesel engine efficiency?

Knock.

= NOx (toxic gas - regulated)

The DI gasoline engine still operates in the same way as before.
 

Fuel is injected early enough to result in a homogenous fuel/air mix well before top dead center. The ignition sequence is initiated by the spark plug. Too high of a compression ratio results in pre-ignition and detonation (knock).

This differs from a diesel which injects fuel into the hot air pocket caused by the high compression at a time that allows a flame front to work to advantage.

There has been some research into fuel injection of gasoline or some other combustible just as the spark is fired to initiate the flame front. However, this results in diesel like particulates ( soot) emissions and NOx because of the non-homogenous flame front. I cannot recall the company from Australia that has contracted with the USA air force to develop such an engine for the drone fleets which still use av-gas and result in a logistical fuel anomaly.

There has been talk out of the EPA about the possibility of adding another fuel grade for gasoline that has a much higher octane so that manufacturers have the ability to more aggressively design Direct Injection systems to provide more power and efficiency in a particular engine. The sticking point is the 30 % ethanol content and how that is to be provided.

All in all, your line of thinking is a good one as more avenues of efficiency are opened up by direct injection of gasoline.

Orbital Engines? That company have been working in a Diesel-like compression ignition for gassers, the HCCI (Homogeneous Charge Compression Ignition) in partnership with GM.