This is like saying that Trump is a better president than Obama because the economy's doing better now than when Obama was first in charge. As if we can be so simple as to think that Trump made the economy be so good and Obama made it be so bad.
In 199419 Democratic Senate seats were up for grabs. They lost 6. (they lost 2 in special elections)
In 2010 16 Democratic Senate seats were up for grabs.
In 2018 8 Republican Senate seats were up for grabs.
Yeah, let's pretend that Trump is amazing. He got through this election by simply not having difficult seats come up, whereas both Clinton and Obama had difficult seats.
In 1994 Clinton lost seats because:
Arizona had an incumbent retire. Seat went from Dem to Rep
Maine had an incumbent retire. Seat went from Dem to Rep
Michigan had an incumbent retire. Seat went from Dem to Rep
Ohio had an incumbent retire. Seat went from Dem to Rep
Pennsylvania saw the incumbent defeated
Tennessee saw the incumbent defeated.
In the special elections Tennessee also went Republican. Al Gore become Vice President and the "incumbent" who had taken over from Gore retired.
Oklahoma Democratic Senator resigned and the Republicans won.
I'm struggling to see how this was 9 seats though. The information seems a little out of balance.
But basically only two incumbents lost their seats in 1994.
In 2010
Arkansas Democratic incumbent lost.
Illinois Democratic incumbent retired. Republican gain.
Indiana Democratic incumbent retired, Republican gain.
North Dakota Democratic incumbent retired, Republican gain.
Pennsylvania Democratic incumbent lost the primaries, didn't run, Republican gain.
Wisconsin Democratic incumbent lost the election.
In 2018
Arizona Republican incumbent retired, Democrat gain (I think).
Nevada Republican incumbent lost the election.
Tennessee and Utah both had incumbents retire and both were held.
Trump had it easy. Had it been a different Senate cycle, had he not had a booming economy, had things been different for Trump, would he have done so well?
Almost certainly not.