Well don't forget that every month we are receiving new hero, and itself is making rolling specific heroes more and more difficult.
Some time ago it was easier to find specific hero, as there were less of them overall and chances to roll specific legendary hero were higher (maybe except SK for me ). But don't get me wrong. When you are rolling gems, game is calculating chances for rolling green, blue and purple hero. When purple hero is chosen then game again is calculating chances for specific hero, and this is what i meant. Earlier there were less heroes, so chances were higher. Now there is much more heroes, so chances are lower to keep 100%. Simple math.
But don't worry. On main acc i have also noticed there are less legends rolled with high amount of gems. And if on smurf i didn't had different situation i would agree with you. But like i said on smurf last time i rolled more legends than on main, and it was like 1,5k gems for legend (and i only found 1 new hero for last 3 months there). While on main it was like 3-4k gems for legend.
So i can only say that it is totally random thing, on one acc you can roll legend left and right even with free rolls, and on other you can find legend after you roll 5k gems.
And as much as i would also like it to happen, i don't think IGG will change that anytime soon. Unless they change rolling system.

The more Heroes that get added means the more diluted the Hero Pool becomes.
By todays standards, 15k gems is very little, perhaps save up a lot more and roll Heroes on the day when there is a Hero Collector event on.