He had a broad platform of membership support, but he was only able to get into that position due to the people who backed him within the PLP, the party machinery and his (undisclosed until after the ballot) financial backers, and none of them would settle for anything less than a purge and junking the 2017 and 2019 manifestos and the 10 pledges on which he stood.
He knew who he was getting into bed with, and he knew that they’d undermine him if he didn’t follow their priorities.