Email is a push-oriented medium—notices comes to the (potential) customer.

Twitter, OTOH, is a pull-oriented medium—the (potential) customer has to remember to check the notices from your account (and you post regularly, so it is not clear which tweets the customer should really pay attention to).

It goes without saying that email is far more effective since you already have the customer’s attention, also you can do long form writing in a way that doesn’t involve extra clicks & URL shortener redirects—avoidable friction—as you would with Twitter as your delivery method.