Disclaimer: I'm speculating on the reason: There may not be a standardized URL scheme for mapping applications whereas both e-mail and browsers have it already (http(s): and mailto:). It's possible though! Apple could define a standard and say "that's how you can open maps" but currently it's a moving target as mapping applications support varying features like directions with waypoints, POI searches, general searches, area selections, etc. They'd have to settle on some standard that Apple Maps support which almost is guaranteed to not support several Google Maps features.