Primarily because Visible's compatibility checker is neither accurate nor complete. Depending on model and how old the phone is it may work with Visible. Unfortunately, you need to get by the checker to even place an order for service if bringing your own device.
With that in mind see the following for a potential workaround. Just be advised that should you ever need support from Visible, they'll likely take the easy way out and say the phone isn't compatible if it doesn't pass the checker.
https://community.visible.com/t5/Need-help/Why-is-my-Galaxy-J3-not-compatible/td-p/48776
Also see https://www.visible.com/blog/post/is-your-phone-compatible-with-visible
Personally I feel that as a budget carrier offering with limited support resources, Visible doesn't want to support every phone known to mankind thereby limiting the checker to those it has confirmed to be fully compatible. In an adult world, they'd just warn you that the phone is not guaranteed to work, allow you to order a sim and require you to take full responsibility for it not working but even then some folks would complain.
