Why has the rear bicycle brake changed now?

The kind of brake you are talking about, I've seen it before, and it looks quite strange. Its point of action is on the wheel axle, I am not a student of this, I am not very clear about its principle, but can talk about my practical experience of my opinion.

I ride a bike with what you call conventional brakes. Most of today's bikes use aluminum rims in order to reduce the weight of the body and prevent the rims from rusting, and the downside of aluminum rims is that they are not as hard and tend to change line, especially on bumpy roads. The rear wheel is a load wheel, in this case it is more likely to deform, become uneven, this time brake rear brake, the body will produce violent shaking and rattling, greatly shorten the service life of the axle. The new braking device you mentioned acts on the axle, the contact surface is more uniform, and less likely to produce deformation, increasing the life of the axle. In addition, the traditional brakes will rub against the wheel rim for a long time, increasing the resistance of traveling. I've ridden the new brakes, and compared to the traditional brakes, the braking distance is longer, but the braking is smoother and more comfortable, without the phenomenon of a sharp stop and roll over. Compared to the braking performance, it is still better to have traditional brakes, most of the high-end bikes have traditional brakes, which are safer, and the rims of these bikes are reinforced, and usually don't have the problems I mentioned, they are just quite expensive.

All of the above is a summary of my experience over the years, I hope it can give you some help, you may want to go and experience it for yourself.