I don't think they scale as well as hydraulics. You can make pressure differential and their mechanical advantage works for you with hydraulic. With electric you need bigger hardware for bigger force.
Even in the case of EPS system, certain packaging precludes their use. BMW 5 series for example I believe uses EPS on V8, RWD cars. But Inline 6 and AWD cars cannot package the EPS motor.
Granted pimento's point is true, as some of the hurdle may be overcome given improvement in technology...
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