Because for the people that will use the mentor system for more than achievements, it's not far-fetched at all to imagine that they'll keep in contact with the people that they were mentoring. I am aware that it counts regardless, but having the mentor system will make it much easier for people seeking help to see who those willing to help are, so it's not outside the bounds of possibility that some of these players will ask questions about things that can happen outside their 40 hour green-leaf period. Being a lv 60 in every role will better suit the mentor in order to answer such questions.
Think of it like this, if you were designing a mentor system, would you make the requirements something that barely gives potential mentors the tools to get by and answer questions in very broad and vague terms, or make the requirements a little tougher in order to better improve the odds that a mentor can answer any question pertaining to a role? I am aware that being a level 60 in no way makes you an expert, but it's the minimum requirement for you to be an expert. I'm not sure I'm wording this as best as I can, so my apologies for that, but when you see a lv 60 player, there are 2 possibilities: they can either be really good at their job and be able to explain specific questions, or they can't, as opposed to seeing a lv 30 player, where the answer automatically becomes the latter.
Moreover, I mentioned in a previous post how it's very rare to see people who play all 3 roles have just one of each (one tank, one DPS, and one healer), and more often than not, (or every time, to be honest, I've yet to see a person who chose to play all 3 roles just have one of each at least 6 months after a level cap increase), making that a requirement greatly heightens the odds that a mentor can answer a more job-specific question, as people who have one of each role at 60 have higher odds of having other jobs at 60 as well.
What I'm trying to get at is that a lot of players are failing to see the mentor system, or rather, what might happen after the first 40 hours, and the development team more than likely saw that coming and decided to implement requirements that will make a mentor much more capable in helping others during their green leaf period, and after as well.