Many possible reasons. If Russia is behind this, it's a great proof of concept test of low traceable weapon that can target and disable high value targets of relatively high profile.
Yep, you test it on your own homeless people, so if you fuck up, noone finds out. If you target a powerfull enemy, you do that intentionally. And with all the speculation about microwaves, noone ever though to actually measure/detect them? EE collage students could do that, ham operators do that (and triangulate them) for fun and competition, and the USA/CIA/... couldn't?
Sometimes with these things I wonder if it might not be a marketing demonstration, sort of like how DDOS as a service providers used to try to take down GitHub to demonstrate their power to potential clients.
"Look at what we did to the US diplomatic corps! For only a few million or a few suitcases of cocaine you too can have this power to use against dissidents in your own country!"
So these aren't intended as tests, they are either intended as attacks or else just something that USG employees get for other reasons.