It really does have to do with the number of maintainers. A huge part of the work is not just building a package manager, but coordinating many stakeholders to all use it. That would also mean building a package manager that covers all of those use cases from day one - what's the point of trying to make people use something that doesn't meet their needs. These things are a huge amount of work. Unsurprisingly, other widely used languages have the same situation - dozens of build and packaging systems for C/C++, Java, Javascript...