Far fewer expenses with equities bought with credit. Usually much less leverage available with equities. Also you can only deduct borrowing costs if you reasonably expect the investment will generate income (an important test). Most people in equities are positively geared so any changes would be immaterial to most investors.
And i agree, restricting NG to new properties may not actually lead to greater investment in new dwellings (I doubt it would), but it would lead to less demand overall (and therefore prices). As Mal points out, it's supply and demand. If say Shorten got in and restricted NG but existing arrangements were grandfathered, those sitting on NGd properties would probably be looking at sluggish demand and therefore be looking at lower expected returns, which would lead to supply. Whether rents increase or not is irrelevant as landlords are likely charging close to what the market can withstand anyway.
The only path to affordability is wages growing faster than prices.
Mal has lost most of the young voters on this one.
