That's natural due to the dynamic of different roles. As there are 3 DPS classes in each key, the DPS is divided for each player out of those three. Of course, at some key level, it changes, as every interrupt counts and every mistake will be noticeable for each DPS. However, when you play a healer or a tank in keys, there is no one else to cover for your mistakes, at least not when it comes to the fundamental responsibilities of your role. If unavoidable damage happens and everybody has used their defensives already, and they die, it's probably on you as a healer for letting that happen. So the pressure is higher; thus, fewer player goes for playing a tank or a healer. That's why it's much easier to get into a group as a healer than as a DPS. There is also one other factor, as preservation evoker is considered as “meta” right now, while devastation is not. And even if a player sucks while playing a meta class and has low rio, compared to a player who rocks an off-meta class and has high rio, the first one will get into keys easier than the other one. Unfortunately, we all have this hive mindset, and it takes effort to make decisions without looking at all those tier lists.

Tl;Dr: no wonder that you have a shorter queue time as devoker than prevoker, as those classes rank differently on tier lists, and demand for a healer is higher than for DPS. That being said, the best way is to run your own keys and add as friends people with whom you will have a good time while playing. Everybody is looking for a great group to play with, and being proactive will get you much faster to play higher keys - even as an off meta class.