Like so many, I had also been hoping for a more epic scale for Legion . But let's assist one another through the stages of grief towards acceptance.
There are decent reasons why Legion should be at the skirmish scale, rather than at the mass epic combat scale.
1. Figure compatibility with Imperial Assault .
While I've also seen that the miniatures are a little off, it doesn't strike me that they are so prohibitively off that you cannot cross-use them. People with IA collections can use their minis for Legion , and also integrate Legion miniatures into their games of Imperial Assault . If you think that the <10% difference is prohibitive, that's on you.
2. Nature of the Galactic Civil War.
The Galactic Civil War simply is not like the US Civil War, WWI or WWII where regular uniformed armies face off against one another, with pitched epic battles determining the outcome of the war. It is about a galactic military hegemon fighting against a small insurgency, whose uniformed military only has the symbols of a legitimate military force.
3. Skirmish-level games are probably just more popular.
I am not sure of the measured accuracy of this, but I'm guessing that games at the scale of Legion are probably just more attractive to most players. GW tried both, and 40K just always seemed to stay on top of whatever Epic stuff they were doing. Ultimately, FFG wants to produce a product which more people will buy. Those of us who wanted something more epic just need to realize that we're probably in the (vocal) minority.
The issue, I think, with both Legion (and Armada ) is that they are named in order to sell more than they are named to accurately portray the scales of the games. I do think that they might have preserved the term Legion for a possible future game of fitting scale, but I assume FFG's marketing department knows what it's doing.