I've launched two cannons into orbit, but it's showing I don't have any available. I tried launching one to a new space platform and it did nothing when it got there, I'm confused what I'm doing wrong.

build the cannon on a space platform, not just send it there.

Why can I make the cannon on the ground if I can't send it to space and need to make it on the platform?

There's quite a few vanilla recipes that are restricted to space-only, so why is this one not space-only?

@Food2411 read the instructions on info page or in FAQ

@R4d6 what advantage would it have if it were space-only?

@Kuxynator Well, it would prevent me from making three Ion Cannons MK2 on Nauvis and then being unable to launch them into space to place on the space platform.

a) reading the description would also prevent you from crafting in the wrong place ;-)
b) if your rocket was bigger, the location would be restricted for nothing
c) quite pragmatic reason, it would take my time to implement this and I don't see the added value because a) and b)
d) as soon as it is restricted, someone wonders why it is restricted