You lack the capacity for rational thought. When has this ever happened before? Presidents travel to foreign countries constantly, when have they ever threatened those countries to get them to fire "corrupt" government officials? Depending upon who you believe we may occasionally assassinate guys we don't like, and we will back political rivals and funnel weapons and money. But when have we ever gone up to country and said "hey, you know that aid money/weapons deal/treaty/etc. we agreed to? Yeah, we're not gonna do that unless you fire XYZ from your administration."
It doesn't happen. And if there was ever a time to start doing it, would you pick a random ass prosecutor whose "corrupt" activities have basically zero bearing on US policy or interests?
Here's a another free clue since you seem to be lacking one. There are thousands, maybe MILLIONS of corrupt government officials around the world. People who steal from their constituents, make false arrests, take bribes, etc. Some of it impacts us a whole hell of a lot more than anything in Ukraine (like for instance the guys making it extra hard for US businesses to succeed in China, or the inspectors that let them put poisonous chemicals in baby formula). If outcry about corrupt politicians was a concern of the US government, wouldn't we prioritize something like warlords who steal food aid, or when they urge genocide against groups within their countries.
No, apparently the last straw is a man so corrupt that there is literally no information available about his specific corrupt actions. His wikipedia page simply mentions that there was one failed vote in the ukraine parliament to have him removed and that "Representatives of the EU and the United States pressed Poroshenko for his removal,
[5] as did the World Bank and International Monetary Fund.", though the attribution of that quote points to a washington post article that provides the same one sentence analysis with no sources provided at all. Well sound the alarms, that sounds like a scumdog that needs immediate american intervention if I ever heard of one.