Yes, it's mainly because the Senate has fewer members and the current Republican-controlled Senate only has a slim majority.
- Majority (52%): 52 Republicans1
- Minority (48%): 46 Democrats + 2 Independents (caucus with Democrats)
Conversely, the House has more members and the Republican-controlled House has a bigger majority, so they can afford to lose a few votes and still pass a bill.
- Majority (55%): 239 Republicans
- Minority (45%): 193 Democrats
Over the past year, it has shown that it is more difficult to pass bills2 in the Senate. For instance, the American Health Care Act was passed in the House on a 217–213 vote, but it could not pass in the Senate. Despite opposition from all Democrats and 20 Republicans (8% of Republicans) voted against it, the bill could still pass the House. This would not pass, if 8% of Republicans in the Senate opposed it.
So, this shows that the Senate cannot lose more than 2 votes on any bill they wish to pass by a simple majority. Getting all 52 votes is difficult due to the fact that it is essential to convince moderate Republicans in the Senate to support any piece of legislation.
As FiveThirtyEight elaborates:
In five of those six instances (all but one that rolled back a regulation created by the Consumer Financial Protection Bureau), the two Republican “no” votes were Maine’s Susan Collins and Alaska’s Lisa Murkowski. This is not surprising. Both senators are among the five Republicans who most often break with the Trump administration’s position according to FiveThirtyEight’s Trump Score. (Collins is the GOP senator who votes against Republicans most often.) And of course, Collins and Murkowski were two key opponents of the GOP’s push to repeal Obamacare.
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Also keep an eye on the possibility of Collins or Murkowski joining with Tennessee’s Bob Corker or Arizona’s Jeff Flake or McCain, the anti-Trump trio that has more political freedom than most members because none of them are likely to face Republican voters again. (Corker and Flake are retiring in 2018, and McCain has been diagnosed with brain cancer.)
1 When Alabama Senator-elect Doug Jones is seated, this will further decrease the Republican majority by 1, making it 51–49.
2 Note that only judicial nominations and bills that go through the reconciliation process can pass by a simple majority. Read more about this here.