what exactly makes it specifically for manga drawing
Workflow and tools. I mean, almost all the art programs are viable tools to draw, but some lend themselves to faster learning curves than others. Why spend months learning Photoshop by YouTube or via trial and error, when you can get down to business in like, a week, using tools dedicated to your task. You could argue that learning Photoshop may be beneficial for doing more than just one type of task, but not everyone wants to/needs to learn that. I like Photoshop for touchups/after effects, or other specialized tasks that aren't present in Sai. Problem is that PS is generally a clunky program (definitely slow to start up, even with my rather beefy comp) and the line smoothing is non-existent (I had to purchase a separate program to smooth my lines). I use Sai to do sketching, linework, and shading (I LOVE the watercolor tool). But Sai isn't a powerful interface, and lacks some really nice workflow features Photoshop has. I haven't tried Clip Studio, but I wager its the best of both worlds (Photoshop and Sai).
