I decided to answer despite PM's comment with the idea that I can always convert this into a comment is PM wants me to.
There are four possible reasons behind GF’s logic in only creating 69 new Pokemon. First off is that they introduced a new type, fairy. Be limiting the amount of new Pokemon, and having a variety of fairy types to choose from, it almost guarantees that every player will have a fairy type and get to try it out. So reason 1: They wanted to promote the fairy type, and yet not have so many that the hype over fairy types dies down before the next Gen. The other reason is Mega-Evolutions. If we have 150 new Pokemon to choose from, then (same basic idea behind reason 1) people will be more likely to have a Mega-Evo on their team. Also by having the amount of Megas limited to only a few favorites, this causes people to become impatient for the next Gen to see if their favorite Pokemon will have a Mega.
There’s also a possible third reason. By having 69 new Pokemon, it allows them some leeway if they decide to go for a third game, like Pokemon Z. I know there are rumors but I have not been keeping up on them, but this is a definite possibility for the low number of 69 new Pokemon.
And the fourth possible reason is that GF simply didn’t have the imagination for 150 new Pokemon, so they created 69, and brought in some of the older Pokemon. I believe this reason has some validity because of the appearance of Caterpie, Weedle, and Pikachu in the first wild Pokemon route, which is in a forest, and very similar to FR/LG.
As for the actual number 69, well Game Freak is just trying to connect with the older players.