Why did portraiture became a staple of commercial photography in the nineteenth century?

Before, only the upper class could afford a painting by which to immortalize themselves, and perhaps only they had the time to sit for hours at a time for weeks or months.

Photography provided the same type of immortality for the less well-to-do, and it certainly took less time. Therein lies the reason for the explosion of it's popularity. Depending on one's outlook, the photo was either more true to life and character than a painting, or a poor substitute. I suppose the latter needed a bit of flattering from the artist's brush!