The availability zones in us-east-1 have unique legacy problems related to companies that would select one AZ and deploy all their huge amount of infrastructure to it. IIRC, Netflix did this, for one example.
Every company that did this was picking us-east-1a specifically, because it was the first AZ in the list. It added up, and now the us-east-1a datacenter can't add capacity fast enough to support the growth of all the companies "stuck" on it (because their existing infra is already deployed there, and they still need to grow.) Effectively, us-east-1a is "full."
Which means, of course, that companies would find out from friends or from AWS errors that us-east-1a is full, and so choose us-east-1b...
AWS fixed this a few years in by making the AZs in a region randomized respective to an AWS root account (so my AWS account's us-east-1a is your AWS account's us-east-1{b,c,d}.) So new companies were better "load balanced" onto the AZs of a region.
But, because those companies still exist and are still growing on the specific DC that was us-east-1a (and to a lesser extent us-east-1b), those DCs are still full. So, for any given AWS account, one-and-a-half of the AZs in us-east-1 will be hard to deploy anything to.
Suggestions:
• for greenfield projects, just use us-east-2.
• for projects that need low-latency links to things that are already deployed within us-east-1, run some reservation actions to see how much capacity you can grab within each us-east-1 AZ, which will let you determine which of your AZs map to the DCs previously known as us-east-1a and us-east-1b. Then, use the other ones. (Or, if you're paying for business-level support, just ask AWS staff which AZs those are for your account; they'll probably be happy to tell you.)