It's hard to theorize because Blizzard made so few tweaks to the game. I can't speak on "fun", but before 2/2/2 the problem was
supports were way too powerful (Overwatch has always had a problem with supports being too powerful). The GOATS meta was a reflection of this where you were borderline throwing if you chose DPS. It's telling how many metas were driven by a support that was too powerful (Brigette; GOATS, Moth meta; Mercy, Nanoblade/Beyblade; Ana). Having too powerful supports means that ultimates are the only thing that kills anyone.
Of course Blizzard had a challenge designing supports - if they were too weak they were unfun to play but if they are too strong then your entire dedicated player base complains for months.
Ultimately I think the problem was the lack of diversity among the other supports/tanks. The relatively few amount of tanks/supports ultimately meant that the community would quickly coealesce on an optimal tank setup (Rein/zarya, Winston/dva, etc), and as a tank/support player it became boring to play because picking an off-meta tank would be seen as a throw. There's little that is worse than queing to play your favorite tank and the other 5 players yelling at you to switch to $meta.