Because Grimm was guaranteed money. Mythic weapons would have dropped much lower if those hundreds of grim eggs had been used to open thousands of locked crates (1grimm = 250plat. 250plat x 100eggs = 25k plat = 1,666crates and 10plat.)

Let's assume 100 Grimm were bought (I know many more were) and that the odds of looting a mythic weapon is 1:100crates (your chances were higher mind you). That would be 16 weapons. Now to just get rid of any skepticism, let's say only 12 more were looted, 4 per each class.

4 weapons more per class for every 100 Grimm eggs bought.

Take the number of Grimm eggs you think were bought, divide by 100 and multiply by 4. That is how many more of that weapon, for your class, could have been introduced to the market.

Ex: I think 350 were bought (I think there were more, just making a point). I would use this equation~
350 / 100 x 4 =
3.5 x 4 =
14 glaives