What caused spam to be such an issue?(my logical answers)
Map design is 1 of the 3 reasons why in my opinion: lots of closed spaces+lots of people together=lots of unnecessary damage and cheeky annoying grenade kills.
The fact that explosives have only a very short cooldown. Probably Dice needed to take inspiration from the battlefield series(which I'm investing lots of my time during quarantine) and needed to do something like in Battlefield 1 that you can have 1 grenade or 2 smokes if you chose them instead and once used (if I am correct) they can only be resupplied by a support player who dropped the ammo pouch/crate and that takes quite a lot of time.
The heavy class in particular is super non-balanced.And I am guilty of using it so yeah, Barrage gives you 3 mini-grenades and either the impact grenade or detonate charge is an 4'th grenade and you can speed up the coolodown with resourceful.
Prob. could think of more,feel free to give more reasons.