Yeah, they moved all trash EXP to the boss, so you'll get nothing on the way but a bigger bite with each boss you defeat.

As for the why? I dunno, actually. I don't see anyone abusing "let me farm the first two packs for exp and then bail" or anything like that. Maybe as a discouragement system for people to just drop, since they'd have wasted their time for nothing if they don't clear a boss first? Idk, like I said, I haven't seen anyone abusing that.