The person is probably most familiar with their former products and can both advocate on merit (i.e., they happen to know about obscure features that no-one knows about for other options) and is more comfortable for them. Knowing people (and being their former boss) probably helps them out, too, which makes it easier to pull things towards their former projects).
But yeah - it doesn't seem like this should happen as often as it does.
But yeah - it doesn't seem like this should happen as often as it does.