Most Situations
In most situations the greatsword or the greataxe is both more common and more useful, as they both do more damage on a typical hit or against creatures immune to crits.
So we aren't talking about making a feat to improve one of the best weapons. We are talking about improving the scythe, a rarely picked weapon, which is statistically about equal with other weapons.
As to "why", well the scythe is an exceedingly big weapon and using its leverage in different ways or adjusting your stance may allow you to achieve different affects. Designing flavor for a game mechanic is rarely difficult.
I think the intention is that a +1 Keen Scythe wielded by someone with the Improve Critical - Scythe and Scythe Mastery feats would have the option of the following every round:
18-20/x4
12-20/x2
While this is a nice option, it only applies to one weapon, and is not strictly speaking very powerful. You can account for high-AC opponents by switching to the larger crit, or wade through mooks with the lower crit. Big deal, the character spent a feat and is fairly unique.
Consider the difference between a hypothetical non-exotic Spiked Chain and its exotic counterpart. For one feat, you have the option of retracting reach and bonuses to disarm and trip. For one feat, the scythe master has the option to change his critical range.
My opinion: It won't break the game, and it will probably enhance someone's entertainment, so go with it.
Technik