Escape Zeppelin has stated the tradition, and he has indicated one of the reasons this hasn't happened, but the reason is basically tradition.
Presidential electors can vote for literally anyone who is a native born US citizen, alive, and over 35 years of age.* That is the constitutional requirements. They are not limited to the Democratic and Republican nominees, or the top two popular vote getters, and there are lots of historical examples of electors voting otherwise, including in the last election.
However, to a degree amazing to me, I keep running into people thinking that it is literally unconstitutional to vote for anyone other than the Democratic or Republican nominee, and this belief is widespread enough that the system spreads.
As for bribery, everyone has their price.
If someone bribed 270 electors to vote for him for President and his wife for Vice President, there is constitutionally pretty much nothing anyone can do to prevent him from being sworn in. Congress can invalidate the election of the electors in the relevant states, though I don't think technically an elector either being "faithless" or accepting a bribe would invalidate the election. Congress would then have to pick the President and Vice President themselves, and the result would be a mess. Also, if bribing electors, which seems to have never happened, became a norm, bribing members of Congress, which has definitely happened, would be a norm too. The other alternative would be to let the new President take office and then impeach him immediately, along with the Veep, and runs into much the same problem.
So I think this is a valid POD if this developed in the nineteenth century, when constitutional norms were still gelling. There is a precedent in 18th century Polish elections, which the framers worried about happening in the US.
* A few electors in 1872 attempted to vote for a dead man, and their votes were disqualified, so historically electors haven't even been able to stay within these restrictions.