The reason Georgism fell out of favor on the left is because Marxism already develops beyond where Georgism falls flat. It's certainly broadly appealing, in that liberals can get behind it rather quickly, but it falls short of Marxist economics in completeness, to the point that it doesn't really bother resolving the fundamental problems with capitalist exploitation, centralization, crisis, or production and overproduction, it just focuses on rent.

It's also very difficult to get through, it's a reformist approach that depends on asking those that have full control of the economy to make it less exploitative. That doesn't happen without revolution, at which point you can go far beyond and address core, systemic issues.