Given that both the older games as well as the newest ones are here it's weird that the ones in between aren't here.

With Stronghold 3 Firefly started to love Steam DRM and I think even if they wanted it, it will likely not accepted on GoG because it whas so bad. Legends itself whas not bad but it strangely hit not the taste of the Fans.
For 2 I have no idea why it is not here either. I find whas not as good as 1 or Crusader but not bad either.