Because I've hit a wall with spell learning and I'm at the point where I pretty much need help with most if not all spells (BLU61). If it helps, I have all spells up to 61 (Yes, even Radiant Breath...I hate you, Radiant Breath. Very much.).
Really though, I don't understand why people want an LS for a certain job. Why not just make friends with other blue mages? When I was still kinda new to the game I got invited to a RDM LS and it was pointless. Besides, can't other jobs help you get the spells? Like a certain awesome puppetmaster who loves to help people named georgis.
Really? So you can help me learn spells like Memento Mori off Corses or Body Slam off Dragons (Just kidding. I don't think I want Body Slam. Ever.)? You know what? I'm not entirely sure why I wanted a BLU LS. When I was first playing as BLU I kept hearing about BLU LS this and BLU LS that and they all were helping other BLUs learn spells. In the end I only had help for a few spells. Come to think of it...the BLU LSs that I heard about either fell apart shortly after forming, I got denied entry or I just couldn't find a sackholder. And I would love to take you up on that offer, Bro but I don't play late night anymore. ><
Somehow job specific linkshells don't really last long lives on Titan.
I remember one being around, but breaking pretty quick because like with many other job specific linkshells most people already have a linkshell which keeps them occupied so even if many people carry a pearl they hardly log into the LS and when they do such LS usually is empty.
The only job specific linkshell that has been around for a while was a BST LS, but I'm not sure even that one exists anymore.