Could be several reasons, there was steady economic growth, prices rose in everything.There were sharp jumps in prices of building materials, lands.
During volatile 90s, early 2000s the main way of real estate building was "долевое строительство" or "долевка". Which made possible to build at rampant inflation and the lack of normal credit market. Mortgage was unheard of and people entered as investors ("дольщики") into the contract from the begining of a house construction paying by installments and that let builders go without borrowing. It was a risky market with happening of protraction ("долгострой") or even fraud. Then the government hampered that way taking funds on unfinished constructions from home buyers which led to reducing housing conctruction while the credit market started to divelop giving buyers money but the market offer laggeg.
That's my reckoning.