Why are there next to no interstate business loops in the Northeast?

[Kacie Jane]

Another reason could be that since traditionally there have been few, if any, signed business routes (US or state) in New England, signing Business Interstate routes would not typically be proposed by most of the state DOTs.

NJ has plenty of business US routes, especially for US 1, but no Interstate business routes

That's actually the only one.  Wikipedia says US 322 has a new one where a bypass was built, but it's unsigned.  There's also US 1/9 Truck to avoid the Pulaski Skyway, but that's a truck route, not a business route.  The one in Trenton is the only one that's signed as a US business route.  I'm also pretty sure Route 33 in Freehold is the only state business route.

Not plenty.