What's the official reason why it became so hot in the Kola borehole?
Unfortunately, you intuition about subterranean ground temperatures is incorrect.
Basements and cellars do provide cooler conditions to store perishable items because the near surface rock insulates against surface heat.
Go deeper however and temperature increases. Near the Earth's surface, within the crust, and away from tectonic boundaries, the rate of temperature increase, due to the geothermal gradient is  25–30 °C/km. The geothermal gradient increases with depth.

Most of the heat is due to the decay of naturally radioactive elements,mostly within the mantle.

Heat flows constantly from its sources within Earth to the surface. Total heat loss from Earth is estimated at 44.2 TW.