Short version: because assault rifle assembly is quite not good.
Long version:
Example:
A2 chaingun vs assault chaingun
+4 accuracy both, both use 4 bullets.
I assume all shots hit.
Against enemy with 0 armor one deas (1d6)x4 other one deals (2d5)x2. 4d6 vs 4d5.Really "cool" increase in firepower.
If enemy has armor, then assault is a bit better.
If you have SoB2, you will deal more damage with A2 chaingun because most of the enemies had 2 or less armor.
If you have SoB5 and fire against enemy with 2 armor: (1d6+3)x4 vs (2d5+3)x2. Even "cooler" than without SoB.
Triggerhappy helps a bit, but at the cost of your ammo.
The same applies to plasma rifles, but plasma damage cuts armor in half, so you don't care much about each shot being stronger.
Assault plasma is (2d6)x3, right? You lose the same 6 cells, but you deal less damage.There are much better ways to improve your DPS and use your mods, than using this crippled assembly, which was good in the past.
End of long version.
Bout burst cannon:
1d9 x 8 with +0 accuracy, and maybe one more mod.If you don't have EE2 your shots are not likely to hit anything exvept walls.
With the same EE2 you can do P2T3. 0.6s firetime, same +2 dices.And it's more accurate.
The best DPS setup for plasma rifle (and nuclear too)(assuming you max SoB and TH) is A2T3 for no-EE wieldes and T3 and something for those with EE.
Added: probably i got a bit carried away. Assault rifle assembly is at best when you're completely incompetent with rapid-fire weapons and it becomes worse when you become better.Burst cannon has the same problem,but to a lesser extent.I mean that if you have SoB5 and your burst plasma deals 1d9 it means that quite often bullets will knockback the enemy, and due current implementation most of the time you will knockback your enemy out of your bullet path, so rest of the volley will miss.
It leads to such problems like A1P3 plasma rifle is worse than A1 plasma rifle if you have SoB5.I experienced it myself and was quite confused when i saw enemies surviving two or sometimes even three volleys of (1d10+5)x8.