I now have 3 of them ....apparently one has come just to fit a thermostat 
[serious]
That is because thermostats are electric.
The horny-handed sons of toil who work on gas pipes will sometimes touch water pipes, but rarely the other way round.
Neither of them will touch 'electrics' because that is black magic and only touched by blessed acolytes.
Gas and water practitioners are taught from an early age that electricity will bring them a terrible and fiery death if touched. [/serious]