Why is home heating wiring so low tech?
why central heating systems are not more modern


Because the established systems like Honeywell Y-plan or S-plan have
many benefits ...


Low cost parts, simpler and sold in volume.
lower cost maintenance - heating engineers are used to these arrangements
and know how to diagnose problems.
Fail-safe. A fault in a sensor can't turn on a boiler without turning on a pump.
Compatibility - the traditional programmer has a common backplate
used by most manufacturers. You can snap in a new programmer
without rewiring.
Reliability - most failures are due to wear in pumps and zone-valves, or
due to lack of maintenance of the boiler. 
Internet appliance makers have an appalling record for security, privacy etc.
I can buy parts and accessories for a 30-year old system. There's
no track record for companies like Nest - we don't yet know whether
they will be around in 30 years time or whether they will make
drop-in replacements for current products.
Vladmir Putin's henchmen can't hack your heating to improve GazProm revenue.


So far as I can tell, systems like Nest only have significant benefit if your
lifestyle is erratic and you need to remotely instruct your house to keep cold to save money.


  Why isn't there a central controller


There seems to be a trend where more of the functionality is incorporated into the boiler. For example it is becoming more common for the boiler to control the pump so that it can run-on after the boiler stops, thus removing residual heat from the boiler.



Of course, systems like Nest don't change much of the above, they are designed to slot into those types of system. Often they function as a combined programmer and room-thermostat replacement with additional features.




Hacking Nest




Random example showing industry standard backplate interoperability: