Just tighter controls on ephedrine and pseudoephedrine. Any dumbass can make it in the bathroom easily with homemade lab equipment using ephedrine, so a lot of law and law enforcement crackdowns have centered on that.

I haven’t seen the show in a long time and never finished it, but from what I remember, later in the show they ditch both methods and move on to methylamine. Going that route would also allow production of isolated d-methamphetamine.