Can only port phone # to major carriers?

[AC Question]

I'm getting ready to abandon my grandfathered unlimited data plan with Big Red (Recently jacked up to $95/mo) and move to a lower cost plan. I usually have wifi so I don't need too much data, but I certainly use the internet.

According to Republic Wireless and CREDO, my number can not be ported.

AT&T, T-Mobile, no problem.

What's going on here?