I like tanks. Tanks are great and wonderful. Tanks require thought and effort. I mained tanks all of ACV and they are still my favorite leg type in VD.
Tanks also depend greatly on the player. In a 4 v 4 where you have allies that can tell snipers and melee bots to GTFO a tank can put in serious work with their raw DPS and defenses. If you have two skilled tankers working together and covering for each other it can be very difficult to crack the opposing team without losing multiple members in the process. Tanks are also wonderful carriers of Subcomputers which encourages teammates to stick close and gives them instant lock cannons.
The reason you don't see more tanks, particularly on JP, are Rabi kumo bots, the very heavy poke based metagame and sadly, permaglide.
To put it simply a Rabi kumo bot will beat a tank even if the tank has an ally attempting to help it, unless you get really lucky with a kick, run a pulse cannon or mathuras, or the Rabi user has no idea what they are doing. Honestly, if you are a tank and you keep the opposing teams' melee user occupied for a couple minutes you have done your job.
The other reason is the shift towards a very long range metagame. Its not uncommon to find teams with 3-4 Sawa and/or sniper builds. This takes away one of the tank's best tools. High DPS. The tank ends up running laser cannons or sniper cannons which in turn leaves it more vulnerable to close range attackers or builds with greater mobility (read just about anything with permaglide). That being said, LCTs or SCTs can really put in a lot of work if the user knows what they are doing, and they will win out fire trades against poke builds with the same damage type. (This isn't taking shields into account but I really don't want to get into semantics)
Finally permaglide, which is by far the easiest of the above list to deal with, gives any build the mobility to literally glide around the tank in a circle forever. In ACV a tank could very heavily punish a genbust (to the point where it was game over) but in VD all the opponent has to do is stay in scan mode and keep glideboosting until they escape effective range. The tank will run out of EN 1st since it is the only build which cannot use permaglide.
However, don't let any of the above things I've mentioned discourage you from using tanks. You still have the best armor and DPS in the game (by far) and you can adapt to whatever your team needs by running different cannon types. On top of that learn your boost charge: if you hit something with it, its a free kill and you have the largest kick hitbox in the game.