Not a bad idea and we, (the Industry), have looked at it. Unfortunately biology comes into play there. The Average Alpaca weighs between 135 and 180 pounds. They live for around twenty years and get to maturity in three years. The average Gestation time is eleven and half months to a full year. There is one Cria, (baby), at a time. So herd growth is necessarily slow.
Goats on the other hand can have two birthings a year usually of twins and the grow quickly to maturity. A four to one advantage when producing meat animals or herd growth.
As a type of livestock that has been bred for a specific purpose over 5,000 years, (so I am told), they are admirably suited for producing fiber, (hair, fleece, what-have-you), that is of a quality that is simply amazing for it’s softness, strength, warmth and generally all the good qualities that humans look for in fiber to make into clothes. That means that to make them a meat animal would be to the detriment of their fleece because you want to fatten them up and put on the poundage, which all leads to degraded Fiber production.
I figured it came down to that. There's a reason why no one has successfully domesticated elephants or bears for meat via selective culling - too expensive to feed (relative to the yield) or too large to control.