Each ship in the fleet is capable of destroying a planet on it's own. Why build Starkiller base at all?Apparently it's cheaper to drive a planet around than an entire fleet of starships? Why bring out the ExtraSuperMassive weapon if the SuperMassive weapon will do to quash the
rebelresistance scum? Or possibly the legion of Sith Minions (who may or may not have been spirits of sith past or current living groupies) were still installing the cupholders and air fresheners in the Fleet o' Doom during TFA so they couldn't deploy yet? And per TRoS, Snoke was a (potentially literal) puppet with the Emperor pulling the strings, so it wasn't particularly his call to make to deploy the huge fleet.
(All that being said, I enjoyed TFA and TRoS...)
And, yes, Snoke was just Palpatine's puppet. Palpatine knew he had the tech to put a superweapon on a Star Destroyer. Why build Starkiller base if he already miniaturized the tech?
And before you say "well, maybe the tech wasn't ready yet", one of the points of Star Was canon was supposed to be that tech was relatively stagnant and that the galaxy has been at roughly the same level of tech for thousands of years. Knights of the Old Republic takes place nearly 4,000 years before the current trilogy, and has pretty much the same level of tech.