Why do some cars use electro-hydraulic steering instead of only electric or only hydraulic steering?

Because as the power needed for turning the wheels increases the price and size of the necessary electric motor increases exponentially ie gets very expensive.

However the hydraulic pump and hydraulic motor needed are easily available in many sizes and the price is cheaper than electric. Also consider that 10, 20 or more years ago any electric motor system was rarer and more expensive while hydraulics was & is well-established with many sources for pumps, hoses, motors and people who understand how to test and fix them. Try finding how many people can fix a vfd for an electric motor...

Many vehicles have powerful hydraulic steering systems turning wheels that weigh the same or more than a car... Providing the electric power means alternators and expensive motor controllers and heavy cables. Much easier with hydraulics.

Also the Construction and Use Regulations vary for the use of the vehicle. Some agricultural machinery does not need a mechanical connection between the steering wheel and turning wheels, relying on hydraulic fluid to provide the connection. But vehicles designed for road use (cars, trucks etc) must have a physical connection (all metal linkages etc) and electro or hydraulic systems can reduce the effort required from the driver