1.They're 2 divisions apart. 2.Golovkin has no title at 154. (Hes never weighed 154 in his entire pro career) 3.They fight on different networks Theres 3 I can think off the top of my head.
Mayweather has the frame of a light welterweight / weltherweight, he barely makes 147-148 on most if not all his fights ( except the marquez one, but he didn,t boxe for a long time ). Why should he fight someone who comes fight night, is in 170 ?