I'd guess reading ebooks, even on tablets or phones, makes one-book-reading the path of least resistance. I mostly read multiple because I physically keep the (paper) books near where I read them. I'll read a different book in bed than at my desk than in the living room, because
that's what's there. Some of those (the bedside one, especially) don't tend to travel with me. Others might. It's mostly a result of the physicality and locality of the actual books, as objects.
However, if I'm reading a book on my phone & tablet, I'll mostly stick to one until I'm done with it. They way they sync reading status and time-of-last-access across devices encourages it, even with multiple devices.