The final straw was when the FDP noticed they had fallen below 5% in the polls shortly prior to election day and started to desperately beg for lend votes from CDU voters. Angela Merkel's public and blunt response was that anyone who wants see a CDU government led by her should vote for the CDU and not the FDP.
The point is, the CDU probably could have saved the FDP if they had wanted to. But Merkel decided to say: F**k 'em.
Correct me if I'm wrong, but I believe
the Lower Saxony state election earlier that year was the main reason why no substantial CDU to FDP loan voting took place. Based on the polls, more than
half of the FDP votes in Lower Saxony were loan votes. Had the CDU voters stayed with the Union, McAllister would have had a majority on his own, but with the FDP entering the Landtag, the SPD-Greens won by one seat. I think had this not happened, the FDP might have very well have got enough loan votes to get over the hurdle.
