Because the immediate need to get people out of the Kampongs (segregated by race and Chinese dialect Clans) and into (mixed) modern housing is far behind.
Demand for HDB flats are far less than it was in the 1970s/80s so the construction of new block is not as urgent. (At the peak a new flat was completed every 45 min.)
At the same time the demand is for higher quality so many of the early blocks are demolished and New Towns are springing up in areas that was regarded as “too far away” before MRT was available all over the island.
New flats are now offered on “Build to Order” principe to avoid oversupply:
https://www.hdb.gov.sg/residential/buying-a-flat/new/sales-launches/bto-sbf-open-booking
New HDB-flats are only available to Singaporeans that is married, or singles over 35 years of age. This has resulted in “kids” staying with their parents for much long than is healthy.
At the same time it is encourage for young couples’ to buy flats near their parents so they can look after them longer. (Extra subsidize are offered)
PS> Young Singaporeans no longer ask; “Will you marry me?”, they now ask; “Shall we apply for a HDB-flat together?”. 