Because of a defense mechanism put in place by your ego called a veil. This “veil” has a few purposes, but one of which is to block remembrances of “former” things or the remembrance of “past” incarnations. This is for your benefit as to not allow past trauma or belief systems to affect your current learning curriculum. It allows you to have a fresh start if you will. 

This “veil” was put in place by a collective soul group (that you might have been a part of) that helped create this dimension in the “beginning” which was created out of what some call the dream realm. This dream realm —you could say— is what our Universe “Banged” out of. And around 2012 this collective soul group agreed to alter the frequency of this “veil” a little allowing the potential for higher streams of consciousness to propel us into the next age. This next age is what some call the age of Aquarius.

To your second question: nothing is ever forgotten just not allowed to be remembered for a time. Believe it or not, not remembering your past incarnations takes a lot of energy on the part of your higher Self to block this remembrance that’s why these lifetimes are only so long. Have confidence that you will remember more of this life after this life than you do during this life! Cool huh!!