Hmm as far as I know, they made special appearances because ;
1) Aiba and Ohno went to the ‘Yoiko no Mikata’ filming location for C no Arashi. They asked Sho’s co-actresses (and those kids and their mothers) about things that they wanted Sho to stop doing and stuff (Well CnA stands for CLAIM no Arashi afterall, so).
That’s where Ohno literally ‘begged’ the director to ‘please let us be in this drama too…there are five of us in Arashi…’
LOL I’m sure they did that just for TV, but the director/producer/etc DID agree to what Ohno asked of course. =P
Ohno even wore the CnA suits for his part in the drama, LOL.
2) Jun appeared as Shin because ‘Gokusen Special’ was filmed around the same time that the last episode of Yoiko no Mikata was filmed?
Both dramas were broadcasted by NTV afterall.
We all know that Jyannis is like the boss of the showbiz! Also, some TV stations do favour Arashi
3) Some of the other dramas ;
Because the members appeared in those dramas back-to-back? They even talked about this during the MC part of which-concert-again-i-can’t-remember, but I know they did.
Example : Maou + Ryuusei no Kizuna (TBS), and Ryuusei was aired after Maou. Same time, same slot.
This isn’t ‘unique’ to Arashi members only. Real actors also appear as guests in the dramas that’re shown before theirs, etc etc.
That is…as far as I know. Can’t remember any other ‘reason’ atm.
There’s always an exception to every rule of course.