-- HOW TO GET THE EXPANSION PACK --

[Roenlond]

Because the expansion was part of the Portal 2 ARG.

and that means that only the steam users deserve the dlc?

Valve made Steam, Valve made Portal 2. Without Portal 2 Justine wouldn't even have existed and it is therefore released exclusively to Steam early on. Frictional probably made a deal with Valve to keep it to Steam, we should be glad that we get to see Justine at all