There's a reason. Kevlar cord can be ordered from the factory in a form that will not become embrittled with cyanoacrylate glues. You must be getting your Kevlar cord from someone in the rocket biz who has gotten the right stuff. I wouldn't put Super Glue on Kevlar fishing line. I actually prefer to soak my Kevlar cords in epoxy resin, which has a little flex to it, but a good grade of CyA glue will work just fine. By the way, you can actually sand hardened CyA if you wet-sand it with debonder on the sanding paper. It's hard to conceal a CyA-balsa joint line because the balsa is so much softer than the glue, but wet sanding with debonder will feather the edge out nicely.