iostreams are horrible to use, especially if you want to format your output and they blow up compilation to a mythical degree. People go to great lengths to avoid them and I agree with them.

print/println are based on the fmt library (https://github.com/fmtlib/fmt), which had it's first release in 2015, so it's roughly as old as Rust afaik. It's mainly inspired by Python formatting.

Having per-type formatters is just a logical conclusion of having type-safe formatting.

iostreams are for all kinds of io (which includes stdin/stdout), while fmt is entirely dedicated to formatting strings. Those things are related, but not the same. cout and cerr and such will probably be entirely superseeded by print/println (I hope), but it doesn't make iostreams generally redundant.

println adds a newline and you want to be able to choose, so there is print and println.