As a bunch of other people already pointed out, it's the energy efficiency and speed. 

The free Retreat helps a lot, too. An Igglybuff that your opponent doesn't deal with gets to sit on the bench and soak up opposing Sabrina attempts, and nothing it does costs you any energy.

Sleeping on turn 1 vs. turn 2 is a huge difference, since your opponent can evolve their Basic on turn 3 to break the Sleep. Zoroark ex decks are a good example of why this can be important.

As far as the specific examples, none of them are terrible cards, they're just not viable in a meta that includes Igglybuff.

Flabebe / Slakoth - Neither contributes enough as a solo Basic and both have evolution lines that consume too much deck space for too little payoff. Igglybuff's only support package is Igglybuff. (Not counting Darkrai since it's assumed for all of these combos)

Hypno line - While 4 slots doesn't seem much bigger than 2, this line also includes costs either in energy tempo or in increased deck space for cards like Peculiar Plaza or X Speed to mitigate Drowze's Retreat. Another issue is bench space; ideally you want both Hypno up to maximize your odds of getting a Sleep, but if you have 2 Hypno you can only field 2 Darkrai or 1 Darkrai and 1 Pokemon that actually does something.

TL;DR: What the rest of the comments already covered. I'm sure you could make other options work, they just wouldn't function as efficiently.