The example you linked is not a font but a collection of individual SVG images. The only benefit to using these would possibly be less aliasing on high-resolution mobile devices. For the most part, the example collection actually uses about 3x the bandwidth of the current implementation, and it is missing a number of flags.
I don't see much benefit of compiling our own font file either, as it would use even more bandwidth by downloading all flags at once, even if they are unused. The current approach of separate small images has a lower bandwidth usage due to fewer bytes being requested, and just as any other font-file/SVG could have, the images should have long-lived expirations in a user's cache. The only benefit of a font/image-map would be reduced latency on the initial load of the flags, which is not a problem I have ever noticed.
Additionally, the current implementation theoretically allows admins to invent their own languages with custom font icons, which a static font file would not permit.
At this time, I do not see a compelling reason to make this change.