(1) the automobile which increased the accessibility and productivity of formerly-marginal land, lessening the problem of equitable access to land, and
(2) the introduction of the income tax as an alternative to land value taxes. Notable, land monopolists preferred income taxes because as long as their land is unused or underused they are not producing as much income. The Georgist movement then split between those who thought that income tax was a good enough replacement and those who "knew" it wasn't.
There's also a kind of conspiracy theory which claims that (to a certain extent) neo-classical economics (which developed at about the same time that Georgism become popular) was promoted by wealthy and biased land owners and monopolists at the time with an intent to discredit Henry George and obfuscate the role of land in the economy, folding it under the definition of 'capital' to reduce the factors of production to just two, along with labor.
That’s all Georgism is. When I agree to pay $3000/mo for a crappy studio to be close to a particular park/subway station/employer/school, some large portion of my rent should go to fund the things that enticed me to move there.
There are lots of reasons great ideas may not take off, including the difficulty of grokking them and/or entrenched interests.