Because Gamefreak is retarded.
I'm all for the "Aquatic creature doesn't necessarily need to be Water type" rationale. Things like Pincurchin, Malamar, Eelektross, etc. are totally fine in my book.
But Gropploct was a case where it really did want that Water typing, because:
1. The game is bloated with 10 fully-evolved mono-Fighting types, so more type diversity would have been nice. Poliwrath isn't in and this was before Urshifu and Keldeo, one of which is a dumb kiddie legendary Pokemon (not sure about Urshifu though).
2. STAB Dive would've been great in conjunction with Octolock.
3. Out of all the aquatic non-Water types, this one actually screams "Water type" the loudest thanks to its huge Water movepool, dex entry, and it being sometimes found in the Water.
