This is a bit of a leap, but I feel like they haven't buffed Mobility yet because they know they'd need to buff Recovery afterwards. Ignoring class abilities, Resilience affects total health by up to 8%, damage reduction up to 30%, flinch resistance up to 10% and reduces Stasis break-out damage. Mobility governs walk/strafe speed and jump height by up to 40%. Recovery affects the time it takes to for health regeneration to start, and the rate at which it regenerates; 50% is the maximum difference in the total time to go from near-dead to full health. If Mobility gets extra effects, such as handling scalars, sprint speed boosts and ability animation scalars, then it goes one of two ways. Either, it is a big buff for Hunters like how the Resilience changes buffed Titans, or it is a big nerf to Titans and Warlocks that have low Mobility i.e. all of them. It leads to Titans and Hunters having extra incentives to invest in the stat that affects their class ability while still being able to get the full benefit of the other stat too, while Warlocks could feel hamstrung into using Recovery - especially since Rift is the slowest-charging class ability - and only choosing one of the other two. Recovery could then be buffed and, if not done carefully, could upset the balance the other way. Another issue is that, unlike the other two, Recovery does not scale linearly. The gaps get wider as the tiers increase, meaning that it's not worth using unless you have a high tier; the difference between Tier 0 and 7 is about the same as it is between 7 and 10. Mobility should be buffed because it is a throwaway stat for 2 of the 3 classes - I choose to accept having low Strength when I invest in Discipline, for example, but know there is basically no downside to having low Mobility when investing in Resilience and Recovery - but it's not so simple that buffing it will solve every problem and not create new ones in doing so.