All of the blogs I have ready about breadcrumbs mention that breadcrumbs are used for secondary navigation and shouldn't really act as primary navigation. What is the reason for this convention?
TL;DR Main menu leads the user, while breadcrumbs are following him. They relate like a GPS navigator and a GPS tracker.
Main navigation supports and helps to form user's mental model of what is on the site. It has constant structure and is presented on the same place, with the same information. It's very reliable and consistent mean for navigational tasks.
Breadcrumbs, on the contrast, are dynamic and can be build in a different ways, following the user interaction pathes. So they are dependant on user's behaviour and don't show clearly the site content. They are helpful as a secondary navigation mean.