Why was Arcanum pulled down form the christmas sale?

I was wondering the same thing.

puhhh lucky me i din´t wait to long and brought it several days ago...

maybe it´s just a system falure or did anybody found other games lacking discount now too?

I'm not sure, but I seem to recall there were two sales, more or less. One ended on Christmas, I think?
This may be a result of that. I didn't keep careful track.

http://www.gog.com/en/news/gog_com_holiday_sale_begins

Activision games are on sale until the 26th of December

Hah and I found out the slow way by checking games in the catalogue I could have sworn were on sale. Ah well, so much to play as it is that I'll hopefully catch it next time :)