Same reason why hardly any toy commercials include the line "In stores NOW!" anymore.
They're already there. Hasbro isn't going to try to sell you a new Optimus Prime by placing commercials for it weeks in advance anymore.

Additionally, your other questions have nothing to do with marketing and more to do with the internal workings of particular stores...

It does have a little to do with marketing. Most stores won't carry an item or items if they don't think it will sell well, and it won't sell well if it doesn't have good enough marketing.

I see commercials for PR toys on Cartoon Network and Nick. You just see them on kids channels since kids are the main buyers for toys. Also kids these days have smart phones and know how to use the Internet. They can look up info on the latest info or have their parents help out. Kids are much different now then most of us were in the 70's, 80's, 90's and early 2000's.

That still doesn't explain the quiet releases and stores refusing to sell the items.

I did a stint as a buyer for a store and marketing had exactly 0% influence on whether we purchased/stocked an item. Commercials for individual items rarely factors in.