Because IT is secondary to their primary mission and they don't have a proper dedicated IT team of sufficient size to enable this or a smooth transition if they tried. This is a repeated problem in the federal government. IT is not their mission, and each entity acts like a small-to-medium sized business, even if they're part of a larger scale organization. The US DOD doesn't even have one email system for all services. Each service has its own, and at least until the early 2010s many (most? all?) military bases had their own email servers with military members (and civilians) having to get new email accounts at each new base they worked at (this isn't the case of having a distributed but mutually coordinated system, it was just that each base literally had their own MS Exchange servers running and each maintained them themselves). And that's pretty much the single largest entity in the executive. Now imagine if it was a struggle for them despite a clear mission benefit how much more challenging this is when the smaller agencies are asked to do the same without being provided sufficient personnel and finances to do it, and no leadership (no real leadership) stepping up to create or establish a cross-service solution.