Because BTO prices are tagged to Resale prices also. So resale in severe shortage now, price keep going up, BTO prices also have to follow.
Those who can't afford encouraged to stay far-away places - preferably JB.
So BTO is tagged to Resale price, which is in turn tagged to high valuation by HDB valuers.
Roughly,
Year 2000 HDB 5-room point block costs $250,000.
Year 2005 HDB 5-room point block costs $400,000.
Year 2012 HDB 5-room point block costs $645,000!
Either the S$ has devalued, or HDB flat suffers major inflation %.
HDB can give reasons why they cannot control market pricing.
However, one thing is within their control - they can likewise "inflate" the grants to help offset the bloating of HDB prices.