- Because they work.
- Because there is a new mech in town that can do it, and everyone wants to test the new mech.
- Because LRMs were severely weakened.
The first point is particularly interesting to me.
They work. If they didn't work, people would ditch them quickly.
Why do they work?
1) Convergence.
If you fire multiple weapons with similar properties together, all your weapons will hit the same spot. (Convergence isn't actually 100 % perfect, so sometimes they actually don't hit the same spot. But your chances of hitting the same spot are good.
2) Heat Capacity
Many of these builds require a high heat capacity. 6 PPCs require the ability to deal with 48 heat in one shot. It would be bad to overheat on every alpha.
3) A certain damage threshold is reached.
6 PPCs deal 60 damage. 60 damage is enough to take off most of the armour on an Atlas arm or side torso, a second hit would destroy it. 3 hits would core an Atlas.
4 PPCs and 1 Gauss Rifle deal 55 damage. 2 hits would core a Cataphract.
2 PPCs and 1 Gauss Rifle deal 35 damage. 2 Hits would core a Cicada. 3 Hits would core a Hunchback.
The first 2 are enabling mechanics - these mechanics allow alpha strike builds in the first place and give them advantages over chain-firing and sustained DPS builds.
But the 3rd is probably crucial why they work - the damage you can inflict is really significant, and it doesn't take very long to severely hurt any enemy you face. Because you can kill many mechs in 2 hits, you don't need sustainable firepower. Most players will probably not be good and lucky enough to hit the same mech on the same hit location twice in a row, but with range and possibly jump jets at your aid, you have time to cool off a bit and try again.
And if you have team mates that can do the same and you focus fire a bit, you have a good chance of taking out enemy mechs quickly. And especially teamed up, the damage can come so fast that a defender might not realize that he needs to react and get into cover before it's too late.
How to deal with this I leave as an exercise to the reader. (For now, at least).