... we do not really need more than 100 new Pokémon every time that a new generation rolls around anymore. Looking at it prior to Gen VI, there were 649 perfectly usable Pokémon, with all manner of type combinations, stat spreads, movepool distributions, etc. So why oversaturate the pool of Pokémon when we have more than more than enough to choose from? If you ran a convenience store, and you had 70 loaves of bread in stock, and only needed to keep about 120 on hand, you would not order 150 more loaves - you would probably order about 50 or 60.
Plus, by introducing fewer Pokémon at a time, they can conserve other potential Pokémon concepts for use in later generations, and do not have to spill it all at once.
I admire the enthusiasm of those who wanted more than 72 new Pokémon, and I can understand their disappointment even if I do not share it, but Game Freak is, first and foremost, a business. They have to pay attention to these sorts of things. Hence, it is wise of them to try to find a balance between "economic and beneficial to the company" and "fan-pleasing and enjoyable."