A providers choice, and some may decide the credit check is too risky for example and say no.
No technical reason if you have two telephone lines into a property. Of course no way of knowing what the state of play for the actual telephone line coming into your property is like, but most usually support two pairs.
The USO only applies to the first telephone line to a property and when ordering a second if more work is needed Openreach are free to bill at cost.
Original refusal was probably down to confusion, and someone thinking you meant can I have a second FTTC line over the existing telephone pair coming into the property. Or a business choice by PlusNet to not offer to you even.