The most likely reason for this descision is to allow people to actually play through the new story with the new jobs - yes, you'll still have to go through 10 more levels to be able to do that, but thats still a lot less than starting from 30 and having to gain 40 levels.

Both samurai and redmage also hightlighted one important thing for me: Yes, getting all of those skills at level 50 felt a bit overwhelming in the first few minutes but after that I realised that the game had given me complete, basic rotations already!
Lets face it: Most jobs are extremly unfinished at level 30 and you have no real clue where that class is heading, which skills are actually going to be important etc. How often did you set up a hotbar only to realise 10 levels later that your initial design was grabage because now you got this new skill that has to fit in somewhere? I know that this happend to me far to often (so often that by now I'm looking at all the skills my job will get at max level and plan my hotbars before I start the leveling process alltogether)
Having a class start at a higher level gives you a better feeling and understanding of its core-mechanics right away. It also makes the job seem more intresting and "attractive" if you get more than 3 buttons to hit right away. And on top of that you can play through the new mainstory with one of the new jobs more or less "right away".

I understand that some people enjoy leveling a great deal - and you still get to level your job through 20 levels after all!
But I would argue that low level leveling - in most cases - doesnt really help you understand whats important about your jobs performance at max level. More often the leveling process will be actually "harmful" to that - certain jobs like DRK for example get key-skills that change how they play at level 70 at the moment. By leveling the class you might have learned how to play DRK below 70 - once you reach that, you need to learn how to play DRK at 70... so... you have to learn DRK again.
I dare to say that 30-50 doesnt even help you to really learn your class - it might help you to understand the basic rules of the gameplay (tanks have to tank, healers need to balance healing and dps, dps has to hit stuff) but not the details of your job.

Honestly, samurai and redmage felt so well designed and ready for content because they started at 50 - at least to me. I already had full rotations, I didnt had to gain 5 more levels to get my "3"! Made both jobs a whole lot more appealing to me - and it gave me a better sense to decided wether or not I actually wanted to try and level them.