The 3e book also had a lot more races in it, including ogres, half-ogres, idra, minotaurs, centaurs, and both baaz and kapak draconians.
If someone who only has the 5e book wants to play a big tough strong race, they're outta luck unless the DM allows for a non-native race. And a lot of people like playing big tough strong races.
Unless there's a section that points them at MPMM for minotaurs and centaurs goliaths (reskinned as ogres) and tells them to reskin dragonborn, of course. But other 5e books have been pretty good so far at keeping all the available options in either the setting book or the PHB, not telling people they have to use a third book in order to play the setting properly.