Of course it's harder, but the number of people who have been able to materialize a creditable Cheng when they are unable to get credited for a Lopez (at least not consistently) is very high now. There's no other instance in gymnastics of suddenly being able to do a 1.4 higher D-score vault. That's a greater score difference than going from a single twisting Yurchenko to an Amanar, which of course never happens. In recent years there are now more Chengs appearing out of nowhere than there are Amanars from people who already had a solid double Yurchenko. The point of this all being - the code is deeply flawed and the Cheng is most "bang-for-your-buck" vault to train.