How to use Traffic Manager with Azure Storage Static Website?

I have a Traffic Manager with an endpoint in Azure Storage Static Website. Static website with index.html is up and running on both http and https. Endpoint in traffic manager has status Online.

But when try to reach static website through traffic manager DNS name http://-.trafficmanager.net I get 400 Bad Request error.

It looks like the error comes from Storage website:

The request URI is invalid.
HttpStatusCode: 400
ErrorCode: InvalidUri

Why? How to fix this?

Here is the answer my to own question "How to fix?":

1. Goto Storage Account -> Custom domain configuration

2. Enter the traffic manager DNS name there blahblah.trafficmanager.net and save config

3. Wait for some minutes.

Bingo! Now blahblah.trafficmanager.net shows the static website in storage account.

Note that storage account must support HTTP in order this to work properly.

Storage account -> Configuration -> Secure transfer required Disabled

Still do not have an answer to question "Why?"