Because it has both huge maintenance costs and huge opportunity costs.
In the UK, some signal boxes still use mechanical interlocks. Any extension is crazy expensive, because you basically need artisans to manufacture these.
Digital technology is much cheaper, much more powerful, safer, requires fewer staff, and has been readily available since the 1980s.
Since the 1990s, we have communication systems for constant communication with the cab. Those can increase efficiency and safety very significantly.
Not upgrading is a massive mistake and a massive systematic failure, and it costs money.