The thing is, the technicality on which the Republicans acquitted Trump was that since the trial was unconstitutional in the first place (Trump being out of office once it was held;
here's Ben Sasse calling bullshit on that) was exposed as the nonsense it was by Burr's voting to convict.
Burr voted to declare the trial unconstitutional twice, when Paul moved that it was, and when the Senate voted on it after the first day of arguments.
But he voted to convict, because at that point it didn't matter whether he thought the trial was constitutional or not; the Senate, which serves as trier of both fact and law (incidentally, this is another reason why Roberts, and not Leahy or even Harris, should have presided) had declared that it was, and therefore it was constitutional. So the only question before him was whether Trump did it.
Every single US Senator knows Trump did it, and knows that it's an impeachable offence for which he should have been removed from office (had he still been in office at the time of the trial) and barred from holding office again. But 43 of them were too worried about their re-election chances (not even 43; Portman voted to acquit and he's retiring) to be willing to say, yes, he did it and he can't hold office again.
As I saw it noted, two votes for conviction were from people who are retiring (Burr and Toomey), three are from people who just won in 2020 (Cassidy, Collins and Sasse), one is from someone who has won despite losing the Republican nomination (Murkowski*) and one is from a state that, while it is solidly Republican, doesn't really like Trump (Romney).
*Alaska recently changed its electoral system to have a jungle primary from which the top four advance, followed by an instant-runoff vote among those four. So Murkowski probably figures she has an even better chance of winning than before.