1. Once land is developed it's expensive to undo.
2. Whoever bought the land will have legal rights, it's a pretty fair argument to say "I bought this fair and square from the govt., it's not my fault the fat orange guy does stupid things".
3. Money.

Here's an example from the UK where a planned railway line has been cancelled by a successive opposition govt.

Last year, a transport minister said no land would be sold until the government had decided on an alternative 50-mile railway line. The Independent understands detailed future plans for phase 2 land will be released in the summer by the DfT.

Surplus land will then be sold, with previous owners getting the first chance to buy - but at the current market value, instead of what they sold it for.

Note the bold bit, land usually goes up in value over time and I doubt the UK govt. will have over paid, therefore original owner may not be able to afford it in the future.

https://www.msn.com/en-gb/news/uknews/i-was-forced-to-sell-my-land-for-hs2-leg-days-before-it-was-axed-now-i-fear-i-ll-never-get-it-back/ar-AA1H1XUD