Cost. Not only would it have to be tall enough to clear the masts of the sailboats, but it would probably have to be ADA compliant, making it insanely expensive. The bridge over West St to Stuyvesant HS is not nearly as tall as this would need to be and was scandalously expensive decades ago at $14m and is less than 1/3 the height a Morris Canal bridge would need to be. Let's not even discuss what a drawbridge would entail!

http://www.tribecatrib.com/content/birth-tribeca-bridge-february-1992