It encourages debt and buying larger homes (e.g. the common advice to get the biggest mortgage you can), which doesn't necessarily increase overall home ownership (larger homes means fewer homes). Even if it does encourage home ownership, why is that a social positive worth spending tax money on - especially when we're talking about putting tax money straight into the pockets of the top ~half of the population (the bottom ~half don't own homes at all), and the more expensive your house the more you get? Even with a cap, that's just backwards.