This is because SEBI says so 
Earllier it wasn’t required but since a year SEBI has made it a requirement. Reason’s for the same could be because they want to avoid defaults / short deliveries. Other reason could be because they want to increase the DEMAT participation - so what if the demat accounts lay idle… lol
Demat account is mandatory for delivering stocks.
In intraday you are not taking any delivery of stocks as long as you buy and sell and your orders are squared off.
If sometimes you buy some stock and you could not sell it (some trading halt etc., Current Market Price hitting upper or lower circuit price etc.,), then you are forced to take delivery of those shares.
In that case, where will your shares credit into?
The answer, is your demat account. Although this will happen rare, you need to have a demat account mapped, in order to get delivery of shares.
That is one of the reason NSE has made it mandatory.