Because they follow 8 long years of failed left-wing policy, which takes its toll. Bill Clinton's 1997 Community Re-Investment Act had a catastrophic effect on the housing market which had a catastrophic effect on the economy.
To it took the CRA (which was actually enacted in the 1970's, but never mind) ten years to bring down the economy, and it had nothing to do with the fact that banks not covered by the CRA were making loans to people who couldn't afford them, and then selling the worthless paper as investments?
Guy, you need to stop getting your economic advice from Hate Radio. The 2008 meltdown did not happen because banks were legally required to loan to poor people if they qualified. It happened because all these middle class white people were buying McMansions as investments hoping to flip them in a couple of years.
In addition - economies are intertwined at least to some degree (and in some cases are completely intertwined) so when the left-wing leaders in Cuba, Greece, Britain, France, China, etc. fail, it impacts the U.S. economy.
Except the epicenter of the 2008 meltdown was the US banks. The epicenter of the 1990 meltdown was the S&L Crisis. You could make the argument (if you were capable of making an argument and not just regurgitating whatever shit you hear Rush say yesterday) that 2001 was indeed a global recession - inventories were too high and had to be whittled down -
If you want to go back further, the 1981 recession was caused by the Fed intentionally driving up unemployment to drive down wages and inflation. (That's when they still had that kind of mojo). Reagan supported and went along with this policy because he considered inflation to be the greater evil.
The 1975 recession was caused by the sudden downturn in military spending when Vietnam ended combined with the oil shocks of the 70's.
Not that we would do anything totally sensible like try to get off of oil or redirect that money into building infrastructure, because that would be silly.
Finally - your idea of "in charge" is skewed. You think a Republican in the White House means Republicans are "in charge".
You might have a point. Republicans are amazingly never, ever responsible for when they fuck up. Just ask Trump.