This is precisely it.
WoW has introduced MMO gamers to a style of gameplay where people are not restricted to group grind sessions when it comes to leveling. Where you could go from 1-max entirely on your own.
To enable DPS classes to play solo however, they needed to be given CC abilities to make up for the lack of healing or their general squishyness. Healers on the other hand were stripped off said CC abilities in favour for some light damage so they could pretend to be able to solo.
And this is where WoW has ruined the Holy Trinity model forever.
Let me explain:
Questing is usually done by killing things. The quicker you are able to kill things, the faster you level. DPS classes in the past could kill things pretty quickly, but due to having little to no CC they were limited to weaker foes or had extremely long downtimes due to health/mana regeneration. Then WoW came and added CC to DPS, added drinks to minimise downtimes. Now DPS classes could kill harder mobs without breaking a sweat.
On the other hand, healers and tanks were only very slightly bumped upwards in the DPS department, but have lost other abilities (like the healers' CC/utility). So even though you could in theory solo you were doing so at a very low kill speed AND had a lot of downtime to sit and drink/eat on top of that.
Almost every MMO in WoW's wake adapted the same strategy when balancing classes/roles. Tanks and healers with low DPS and low utility and DPS classes with high utility/high DPS.
And because people did not need a group to level up any more, everyone started specialising their characters towards one goal - being able to solo efficiently and with the least amount of tedium possible. This is one of the reasons there are no healers and/or tanks.
Basically, if one role is fun to play 95% of the time and the other 2 roles are only fun to play in certain conditions (i.e. you have a group to level with and they're not complete idiots) then you will have an abundance of role 1 and a severe lack of roles 2 and 3.
So, TL;DR:
In the old Trinity renditions, everyone was screwed solo. So leveling was a group effort and every role was valuable and represented well enough within the game. WoW changed that around to make ONLY DPS classes fun to play. And due to WoW's success every MMO since then has adopted the same formula.