The reason why we recommend MPL is because it encourages companies to help maintain Scala Platform modules in the future (be it with money, resources or developers). As the Platform is maintained by volunteers and developers that invest their free time into software that everyone benefits from, this is a huge incentive for them. The Scala License and ASL 2.0 are not appropriate for this matter because they don’t contribute to make maintainers’ life better.
That being said, this is only a recommendation. Libraries joining the Platform can have any open source license they want/already have, so long as it’s compatible with any of the other modules’ license in the Platform.