This is because most major stores can take two weeks to update their catalogues. Stores like Spotify and iTunes are dealing with millions of tracks every day, so it’s easy for them to miss some out. If you give your release a two-week timeframe, it allows all the online stores to have your release ready on the date that you want! Some other distributors let you rush your releases through, but if you are invested in building a proper career in music, this isn’t the way to go. You need long lead times to put real release plans into effect. Rushing your releases out is a recipe for disaster.