It's pretty much a technical reason they didn't foresee. Every FATE has a base EXP gain that scales with your level and the base EXP in N.Thanalan FATEs just happens to be larger than it is in HW zones. They can't lower it because that would affect everyone. What they could do is change the formula once you hit level 50, or cap the experience, which they should be able to do, they already did it to tradecraft leves.