Engine limitation.
Although possible set more detailed clouds in FMB using the object Weather, this clouds is only visible in a buble around the player (like grass), in background the sky is always CAVU.
You can fly though clouds in the whole Channel, but will see the France or English coast behind this "local weather" that move with you.
It's CloD!" So CloD never will have a weather like now is possible in Bo'X, although CLoD clouds looks more "pretty".
Bokin topic in ACG explain what is possible do:
http://www.aircombatgroup.co.uk/forum/viewtopic.php?f=5&t=8360http://www.aircombatgroup.co.uk/forum/viewtopic.php?f=5&t=8373&start=10Since until now TF focus was in MP and MP players hate clouds, this remain as released.