Because my opposition to Ebikes is based on preservation of our wild places that have enough increased human presence without opening up technology to make it even easier.
I have been on a horse once or twice as a child 25+ years ago. If i'm going to use horses for backcountry travel it's going to take a serious ****in commitment to become horse competent and that's just to rent horses. I'm going to need a vehicle capable of towing them and it's going to cost a bundle more than renting an Ebike would. Also, If I rent horses, I'm probably limiting camp sites to be near water, i'm going to have to return to them to feed them and tend to them throughout the trip. That can be a huge burden if you're trying to stay on animals. Horse ownership? Now I'd need land or pay significant boarding fees, need trailers, tack, food, vet bills, etc.
I could throw a rented E-Bike on my wife's subaru and be gone. There are lots of people like me who have no business being around horses who would be putting themselves in significant physical and financial danger by using them in the mountains but the same is not true with Ebikes.