Because of safety. For a brand new concept engine, you don't want to have any doubts at all. That means lots of long term testing. Since it will be powering jet

Because of safety. For a brand new concept engine, you don't want to have any doubts at all. That means lots of long term testing. Since it will be powering jetliners, it has to be as close to flawless as possible. And even then, they have to convince the airline companies to use it.