Explosive Weapons- Heat
Electric Weapons- Energy
Physical Weapons- Only focus on dealing large amounts of damage to the players health due to this the physical weapons are put in legendary because of all the pure damage they deal unlike the other types of weapons

My guess why most high level Physical weapons are Legendary items is because they deal a lot more damage than Heat or Energy weapons. If high level Physical weapons would be as available as Heat/Energy weapons are, everybody would use them due to their high damage.

As item boxes go, you do know the percentage of getting Epic, Rare and Legendary items, right?

Regular item box - 50% chance for Rare, 10% chance for Epic and 3% chance for Legendary, cost 12422 credits, you get 3 items.
Silver item box - 60% chance for Rare, 30% chance for Epic and 10% chance for Legendary, cost 90 tokens, you get 5 items.
Gold item box - 20% chance for Rare, 50% chance for Epic and 30% chance for Legendary, cost 140 tokens, you get 5 items.

I tend to collect about 125K of credits and then buy 10 regular item boxes at once. I've seen that if you buy item boxes in loads, you tend to get better items. Once i got 2 Legendary items (Galaxus torso and Stampede legs) during the purchase of my 10 regular item boxes.

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Aeacus

Like Aeacus mentioned, I also save up and buy in bulk and have noticed the same thing. Another good thing about that is anything you get that you don't need or want, you can use it to Fuse your weapons. At 12k for 3 pieces of "junk" is not bad for when you are focused on Fusing your gear. SM credits are ridiculously easy to get [i probley should not have said that - note to Devs: forget that part...lol] so it is not hard and does not take long to get a lot.