Most players have had 2+ years to get candy for things like Lickitung, Aipom, Yanma etc... Many have also been featured in events such Snorunt. Starting to get the point now?
If they kept the 50 candy requirement, most players would've had 1/4 of the Sinnoh dex filled out within seconds of them being released. Seeing how for most players (especially casual players) filling out the dex is the main goal, it's understandable that Niantic increased the candy requirement and added a stone.