There are a few main reasons for me. First, I just think it's a more rewarding experience to the game. To me, the challenge of having to retool my squad and deal with the various competitions to succeed is more worth it. It's more important for me to always strive for excellence than to have a higher quality team. I almost always get promoted anyway, so the only competition tanking helps me in is Associations (more on that later).
Second, tanking prevents me from competing in CL, which is a big source of prize money. Sure, you still get into Cup, but Cup usually has less money, and you get matched up with teams of similar quality anyway. Same thing with League; you're really just falling back into a tougher league after tanking, most often.
Third, if I tank, it brings up my Association's average quality, which brings us harder teams. This is especially true if I have a significantly higher quality than my teammates; I'd be bringing our overall average up, making our Association play other teams with a higher quality, and indirectly hurting my teammates.
Fourth, it kinda messes up the players. One thing I've noticed is that momentum is a REAL thing in this game; if players are doing well and jelling, they are likely to keep rolling until they hit a wall. By tanking and forcing your players to play out of position, or tired, or just benched, the players can't really get into a rhythm of winning. This is especially true if you are tanking in one competition but then actually trying to win in another.
I do see some merits in a tanking system; however, I think there are many reasons why managers like me choose not to tank. Perhaps Nordeus should try to think about offering a way to please tankers, but I think there are some other things that are higher priorities for them at this point in time.
DRAGONS SC
THE PERMANENT END