This is because ever is an adverb that, your example, modifies the adjective 'popular'. There is no need to link an adverb and an adjective with a hyphen as you might with a compound adjective like 'free-range' eggs. This is covered more here Hyphens in compound adjectives.
There is a section on adverbs half way down the right- hand side under the 'Beware' section.
The exception to the adverb rule is when the adverb could be confused as an adjective (e.g. well or fast).