For what possible reason? It is like having 50 reasonable Internets.

To me it's more like 50+ image formats.

Yeah, 50 might be more than is really needed, but there is no reason why several or more can't happily coexist, each specialized in their own way.

Because many different groups bought into many different viable systems in the same time? Isn't that why many countries in the world use their own currencies?

Protection against manipulation and attacks?

Seems like a bit of a double-edged sword to me. On one hand, it becomes less likely that a single party or exploit would be able to damage the entire ecosystem (because distinct currencies aren't likely to have exactly the same flaws). On the other hand, AFAIK, smaller networks are generally easier to attack, making it more feasible that it will happen.

Not necessarily one, but as crypto matures I fully expect consolidation down to a few with each specializing in some aspect. 50 feels too large except for some niche uses, but again this is a bit of uncharted territory.