It takes a two-thirds vote of the chamber (67 out of 100 senators) to convict an impeached president. That’s a far higher threshold than an ordinary vote, and even the typical supermajority requirement in the Senate. And it has never happened in US history (though it likely would have for Nixon, if he hadn’t resigned first).
There are currently 53 Republican senators, so removing Trump would have taken 20 of them to defect. In the end, only Romney voted to convict (on one count).
Impeachment has polled moderately well, and Trump is moderately unpopular. But to inspire mass defections from Senate Republicans, the landscape would need to have been overwhelmingly in favor of impeachment.