Building nukes isn't that expensive. The most expensive part is probably building the enrichment facilities, but that's a one-time investment. Once you have all the material, a nuke isn't that complicated to build. A bunch of students basically designed one that was deemed to be functional.

On the other hand, launching hundreds, possibly thousands of multi-ton projectiles into orbit is extremely expensive. And of course you have to maintain them in space somehow, possibly for decades. Either that or you have to de-orbit and replace them, which would mean regularly bombarding the ocean or some desert ...

It's just not practical. Even if it was I highly doubt it would be cheaper.