Why is blue essence such a pain to get??

So basically,, a few weeks ago i opened a chest n got vel'koz,,sweet right? Well i don't like vel'koz, but i kept him Aside from vel'koz i had garren, mundo n rengar too I re-rolled since i have a thing for assasins n dw lose ,, I've also calculated how much essence i'd get out of those 3 before rerollin n it was 607.. I got a fiora permanent,, not bad i guess.. Maybe i should've dischanted those 3 n waited till i get another key so i can dischant that champ too,, but seriously why is blue essence such a pain?? orange feels so easy to get it's unbelievable! I just really want my rengar lol.