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.
There's been credible international standards since early on. The government can say "use that international standard for government systems, not these (more) proprietary and unstable things".
The government is generally very slow and deliberate about technology recommendations let alone requirements when it comes to generally available software.