Because Biden is the head of state of another nuclear armed nation, who also happens to be the real power of NATO. When the POTUS says "this man cannot remain in power" in a major speech, it becomes the actual policy of the USofA and everyone in it to effect regime change in Russia to remove Putin as head of state. That is very very close to a declaration of war. In fact, one of the rationales that Bush gave for going into Iraq is that Saddam wanted to kill a FORMER president. Consider if Xi made this very statement about Biden, we would be considering it as a prelude to war. The intel services and military take this stuff as gospel from their CIC and then start making plans/steps to actualize it.
It was completely irresponsible to the point of stupidity. Yes, we all might WANT this to be the case, the USA could even be trying in multiple unspoken ways to make this happen but you do not say "cannot remain in power", without there being consequences.
Again, he didnt say, this man SHOULD not be in power in our modern age, nor even that the Russian people should kick him out of power. Instead, he signaled that it is now US policy to remove Putin via any means necessary. Again, this isn't some comedian on late-night television, some bloviating Senator Snort, or even the leader of some tiny backwater like Greenland or a spineless country like Canada - it was the United States, who effects regime change for breakfast and sh**s new "democracies" by lunchtime.
Hopes that helps shed light on things.