Maybe they are waiting X months (or years) after the physical product or they are waiting for monthly sales of physical copies to go below Y? Or maybe it's related to the average temperature of Jupiter, for all I know...

Apples and oranges.Why do HBO hold back the DVD/blu-ray release of "Game of Thrones" when it's the most widely-pirated show out there?
But it isn't an answer you're hearing in the Wotc case: most gamers would love to have the PHB as a book to read and hand out to players etc, and have the e-book to search and reference, as well as copy content from when creating their own house rules and campaigns.
Apples and oranges.
The utility of a rpg book and ebook is related but different.
The utility of a TV show is pretty much the same no matter the distribution (assuming, as in this case, you talk about ways where you own the content, i. e. non streamed non drm content)
My point is: one answer to your HBO question is: because nobody's interested in the disc when you can simply download the exact same stuff.
It's not an apples vs oranges comparison: despite widespread piracy, HBO held back on the legal ways to own their content, for no apparent reason.
I don't understand. There are already high quality pirate PDFs out there. In what way would Wotc lose out by providing the legal alternative?
But that's not really my question. My question is: how can you put forward the argument "piracy worries keep Wotc from PDFs". I can't find any rational sense in it.
Why would Wotc knowingly refrain from making a sale just to prevent what is already out there??
(there can be other explanations, and obviously I put more faith in those. Just genuinely interested in knowing how you can honestly believe in the "to prevent piracy" argument. Thanks)
Agreed.
And yet now even HBO has reversed course...