Sounds like CGL either misjudged demand or couldn't afford to print enough to meet demand to me. Maybe even a little of both I guess.
Considering that most RPGs that had kickstarters recently had around 4 to 5 thousand backers if they were a core book, I'd say that managing to sell 15,000+ copies of a book is a pretty impressive feat in the modern RPG market, so while it is obvious that Catalyst underestimated the demand for SR5, I think there is absolutely no way they could have expected such demand.
Also, with print runs, especially as more people turn to PDF as their only format for purchasing RPG books, you have to be careful not to over-shoot the demand... books left in warehouse = dollars thrown in the garbage.