A huge part of flying training is building confidence in your student so they begin to believe they can actually perform whatever manoeuvre you are teaching them.
As the student progresses, you can increase the difficulty incrementally so that confidence continues to grow.
I can see how an easier surface - tarmac - might be OK for early training but building the confidence to make EOLs to grass is important too.
In the Brit Mil we were very lucky to have keen, able, well motivated and generally confident students - something you might not see so much in a civilian student. Hence all our EOLs were conducted to grass.
I believe training to grass is a far better preparation for what you might be faced with in the real world and the ability to reduce groundspeed to a minimum is more likely to lead to a successful outcome if you end up having to do an EOL onto a less forgiving surface.