For one, the MIT/BSD/GPL licenses are all between 30-40 years old, whereas the MPL is only 8 years old. So that's 4x younger. For another, Mozilla has never r

For one, the MIT/BSD/GPL licenses are all between 30-40 years old, whereas the MPL is only 8 years old. So that's 4x younger. For another, Mozilla has never really tried to "market" the license. It's hard to make something gain traction if you don't really try. And then nobody really cares that much about licensing so if something is "good enough" and does what they want it to then they're going to stick to it. Especially if it means they get to avoid doing research about licenses. And people that do really care about licenses tend to fall into camp permissive or camp copyleft with not a whole lot of in-between.