The reason is simple. Upgrading is allowed i.e. people who buy hdb, then buy a condo, stay in condo and rent out hdb. Don't forget that there is now an additional buyer's stamp duty as a deterrence. Downgrading is also allowed, i.e. people who buy condo. sell it and buy hdb for various reasons. However if people who already have private property are allowed to buy hdb, what is there to stop a person living in a bungalow to buy 5 (or just 1) hdb flats and rent them out?
Sometimes in life, we have to consider what is good public policy, what benefits as many people as possible, what prevents rich people from benefitting at the expense of poor people, and not just look at our own individual situation and circumstances.