I don't know the first thing about train operations - could someone ELI5 why signal failure is a recurring thing in the UK?

After a decade of upgrades the Met line suddenly had extremely frequent signal failure (around 2019ish) and ran a service that felt considerably less reliable than 2007 before upgrades began.

Places like Singapore, Hong Kong and Tokyo run like clockwork. I also feel like the DLR runs better but I don't take it often enough to intimately know its failure rates.

Is it just the curse of legacy infrastructure?