Unlike many machines that are perfectly useful in isolation from others, fax machines must work with other fax machines.
Information
contrast Fax machines must work with other fax machines, whereas many machines work in isolation from others.
causal chain
Fax machines must work with other fax machines. Thus, having lots of incompatible formats limited usefulness. Hence, fax machines weren't as commercially viable.
causal diff-maker
Fax machines weren't as commercially viable until they agreed to adopt a common format.
(adopting a common format led to more commercial viability)
Evaluate
It's hard to know what sort of correct answer to predict, but it will most likely reward our understanding of how incompatible formats limited usefulness/profitability or how a compatible common format helped profitability.