I really thought that it was going to be something like "Why Metroid Prime 4? Why not a new great game like Other M?", then I remembered that nobody likes Other M. So, basically, what Jumpin said.
Series is a series, franchise is a franchise.
Mega Man is a franchise with some series like:
Mega Man 1-10 is a series.
Mega Man X 1-3 is a series.
Zelda games usually don't have numbers because they are not direct sequences. Pokemon games too, you use a different character from a different region.







