No one 'decided' that its just how our stats interact. Crit is our best stat, but you wouldn't want 80% crit and 1% mastery, you get way more value when they are closer. Stat weights are not particularly useful at this point, because once you get to a certain ilvl, around 905ish mastery and crit can be so close that swapping one item can change which is the 'best' stat for you.
The reason 50% crit is generally held as 'enough' is because of how fast our rotation is, ofc u can still get bad rng and have 5 chaos strikes in a row not crit, but in practice enough will to make sure you don't have fury issues and once your rotation is fairly steady mastery directly impacts how much every single chaos strike hits for.
- - - Updated - - -
the main reason people might stack drit higher than that is item drops - for example, if you have a 910 crit trinket and and 870 mastery one its pretty safe to assume the ilvl will win, the comparison between crit/mastery is only really valid at equal or similar ilvls