Because it never really sets and doesn't penetrate the plaster thus forming a thin skin on top of the plaster.
When the moisture from the adhesive hits the PVA it becomes live again and full adhesion of tile to wall will not be attained.
Thus all that is holding up a significant weight of tiles, adhesive and grout is a very thin layer of PVA which not surprisingly sometimes has difficulty holding it all up and voila - it gives up and the tiles are all over the floor.
If not using a self priming adhesive it's far better to use an acrylic primer.
You'll find a much more scientific answer on one of the ahesive manufacturers sites. My memory (failing) tells me it's the Ardex one but I could be wrong but in any event it's out there all the same.
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