One nitpicky detail — Nixon was not impeached. The House Judiciary Committee passed articles of impeachment, and there were enough votes in the House that these articles surely would have passed in a floor vote. And there were more than enough votes in the Senate to convict him. Which is why Barry Goldwater told Nixon that his party no longer supported him, and explained that he was toast. Nixon resigned rather than face impeachment and conviction.
However, you raise an interesting and valid point — Trump’s actions continue adding to the list of impeachable offenses on a frighteningly regular basis, and so far the Republicans are willing to let him get away with it. Even though Trump is trampling all over the Constitution. Even though Hillary would never have been able to get away with a fraction of the outrages Trump has brought us. Even though Trump is openly thumbing his nose at America and essentially daring anyone to do anything about it.
So why indeed are the Republicans just sitting back and shrugging their shoulders? My guess is that they need Trump where he is to sign legislation they want concerning tax cuts for the rich and other goodies they’ve always wanted (remember that they are still trying to destroy everything Obama ever did). Perhaps when everything on their wish list is checked off, they’ll put the hammer down and get rid of him.
But it’s going to be a race to get this done before the midterms. The House is in real danger of going to the Democrats in 2018 because of Trump’s dragging the GOP down. If the Republicans were to impeach now, they might get rid of Trump in enough time to rehab the party before November 2018.