Which is odd to me why they would change it. I mean it's nice to have both options but weird they made it harder. Also not on the topic of CV3, but I didn't know the Japanese version of Belmont's Revenge uses the cross sub-weapon opposed to the axe. Learn something new every day.
Back in the 80s and some of the 90s there was just an arcade-like mentality to make games hard so that you'd be getting your money's worth. Not sure why this philosophy was inconsistent, though: i.e. Ninja Gaiden 1 wasn't toned down in Japan and ramped up overseas, as far as I know.
Castlevania III is tough to learn, but after years of playing it, I find nearly all of it to be an average level of difficulty now. So, having an easier version of it is neat, but it's sort of too late. I've been trained in on the harder version to the point where it doesn't bother me much. (The biggest hurdle being the doppelganger fight and water area preceding it, probably.)
The cross subweapon may not be as strategically useful in spots as the ax in Belmont's Revenge, but in most situations, having played with it now for a little bit, it seems way overpowered. It's just so big and hits twice and covers a far-reaching area. (It also seems like overkill with the fire-spitting whip.) I can see why it might have been swapped out, though it may have just been a censorship thing.