I don't think the government wants to (or possibly is even able to) get involved in defining web standards. Tracking market share seems like the least hands-on

I don't think the government wants to (or possibly is even able to) get involved in defining web standards. Tracking market share seems like the least hands-on way to determine support as it's purely "pull based" - they follow everyone else's lead, they aren't trying to dictate what a "good" browser is. The government is generally very slow and deliberate about technology recommendations let alone requirements when it comes to generally available software.