(Summary below)
Tanks are unpopular because:
1- First, people like the FREEDOM to KILL stuff, not babysit teammates.
2- They also like MOBILITY to FLANK and/or SNIPE to kill stuff with their guns & weapons.
3- For killing they like a good GUN & WEAPON gameplay FEELING, generally a FAST & LETHAL one.
You can’t do that properly with Tanks because:
#1
-Pushing a front-line and/or peeling for teammates may feel like a duty/responsibility or babysitting.
#2
-Well, Tanks are big targets, they are easy to be focused by damage and CC, which (in exchange) is what makes them good to draw attention away from their teammates to work around under less damage pressure (more freedom).-Most of the options we have are slow and also very dependent on supports to ensure they endurance to do their jobs.
-It’s an under pressure and team reliant (less independent) role.
#3
-Most of them lack long range.-Winston has a low damage over time beam weapon; Rein and Sigma have higher damage, but slow fire rate; Orisa, D.Va and Hammond have lower damage, but fast fire rate.
-Only Zarya and Hog get the fast & lethal feeling (with tradeoffs, of course) and that’s why they’re more popular among damage-focused players.
By having 2 tanks in each team at least they can divide/share the pressure and the attention they draw to them and they bring relevance to the role since they cannot be potentially ignored in comparison to what could happen in a solo tank environment (like in the past or future - 132).
But, unfortunately, it is an unpopular role.
Everybody wants tanks on their team, but no one wants to play them.
Summary:
-People like the FREEDOM and MOBILITY to FLANK and/or SNIPE with FAST & LETHAL guns/weapons to KILL stuff, not to babysit teammates.
-Tanks are not popular because pushing a front line or peeling may feel like a duty or babysitting. Most of them have a slow or stationary gameplay (few mobile options). Most of them don’t have a fast & lethal weapon feeling.
-Also, despite the fact that they are more durable, the tradeoffs are that they are bigger targets, easier focused by damage and CC.
It ends up being an under pressure role that also lacks some popular gameplay appealing aspects.