Unfortunately, I don't have time right now (just passing through real quick), but I'd want to take a closer look at how these tags are used.
As words, they are very different things. Maintenance is a phase in the product lifecycle that exists after a system has been deployed. Although systems in a maintenance phase go through some form of requirements engineering, architecture and design updates, implementation, and testing, the way those are handled can be (depending on the circumstances and the type of system being developed) different than a new product that has not been released. Maintainability is a quality aspect of the system under development and applies anywhere in the lifecycle, from requirements through the maintenance phase.
It's possible to have questions about products in the maintenance phase that have nothing to do with maintainability, as well questions about maintainability for products that aren't in a maintenance phase yet.