We have seen a few requests since the launch of Rise of the Hutt Cartel for the Mission Log size to be increased above 25 and I wanted to explain why that limit is currently in place. The Mission Log, just like all other components of the game, directly affects a player's performance in game. The goal in limiting the log to only 25 missions is to help mitigate potential area server hitches and ability lag they may encounter. Simply put, players could see decreased game performance with a Mission Log larger than 25.
Hopefully that helps explain a bit why the limit is what it is. There are currently no plans to change that limit but it is something we will continue to monitor.
So, in short, once again, the game's terrible engine is to blame for such an annoying limitation.