I think they are more popular now because of ECM. I pug and its a huge risk using streaks or LRMs on the A1 because an enemy ECM can completely shut you down and there is no guarantee you will have one on your team. You also can't rely on having Tag on your team and can't take it yourself.
It seems that with ECM out there, the focus is on rushing into close range and brawling. This in turn makes close range, dumb-fire weaponry favourable: Autocannons, SRMs, Medium lasers.
When I play the 6xSRM6 cat I try to stay with an ECM Atlas. Now they can't see me coming and when I'm in striking range they are usually shooting the Atlas instead.
Keep in mind that with a 300XL the build is incredibly tight. It always has heat and ammo problems and can't fit jump jets unless you are willing to downgrade to 2xSRM4s (Which does work too) or a smaller engine (Which then hurts your speed, which you need to get close).
As always, just shoot the ears, or core me right in the face