Ehh…I can see where this is coming from, but there are reasons.

1. Better advertising.
2. More appealing art style, (enhanced by the fact it is in hd)
3. Splatoon being a shooter designed for everyone gives it something appealing while Codename steam was just a Strategy game.
4. A better demo. Codename steam demo gave off things like those long enemy phase wait times that give off a bad omen. Splatoon's didn't really have much fault, and the fact it was treated like an event also helped. (Though, it was of course a stress test, it just happen to also benefit as a great demo).
5. It's very own direct.
6. It's very own amiibo.
7. It was a main thing during the digital even while Codename steam was put to the side in it's own thing that wasn't that open.
8. Splatoon was also playable that e3, which allowed a lot of people to see how cool it was.

Honestly I do want to get Codename STEAM, it basically looks like "X-COM meets steampunk and classic American literature," and that's awesome on concept alone because it's of the "crazy enough to work" variety. Plus it's made by the same people behind Fire Emblem.

Why do I not own it yet? Unfortunately it's just not very high on my gaming priorities list, plus the timing of its launch wasn't very good for me. Splatoon meanwhile was better timed and ranked higher on my priorities list since I was hyped for that game since seeing it at E3 2014, while STEAM just didn't get as much attention from even Nintendo themselves.