SpaceX is finally following through on its promises and has put a lot of the technical risk of its strategy in the past. All that risk was priced into old valuations, so with a now-reliable and frequently-launched vehicle in Falcon 9, and successful follow-through on Falcon Heavy (less of a big deal economically than the media brouhaha makes it out to be), that risk has gone down and so the valuation is going up.
The launch business is a big [1] (but not enormous) one, and SpaceX has gone from around ~15% to ~45% market share in two years [2]. In my understanding, US DoD launches are a small, but particularly-high-margin subset of that market.
[1] $8.8B annually, according to https://www.businesswire.com/news/home/20180116005923/en/Glo...
[2] 2015-2017, according to SpaceX testimony to a Congressional committee: https://www.popularmechanics.com/space/rockets/a27290/one-ch...