Because above all else the democratic party elites are control freaks who wanted to deal with two situations:

1. A chaotic convention that illustrates the neolib-vs-progressive schism in the party, and the risk that an ACTUAL progressive might come out on top instead of a manufactured faux-gressive.

2. The relative ease of transferring state level campaign operations (and more importantly, money) from Biden-Harris to Harris-Walz.

She was the continuity-of-campaign choice. They would like to win, and Harris gave them a chance of winning, but losing control is more terrifying to the party leadership than losing the election.

Because above all else the democratic party elites are control freaks who wanted to avoid two situations:

1. A chaotic convention that illustrates the neolib-vs-progressive schism in the party, and the risk that an ACTUAL progressive might come out on top instead of a manufactured faux-gressive.

2. The relative ease of transferring state level campaign operations (and more importantly, money) from Biden-Harris to Harris-Walz.

She was the continuity-of-campaign choice.