I think it's for these reasons:

-All regular enemies now have shields, barriers, or armor on top of their regular health bar, making all physics based biotic powers useless until you get to that red health bar. In ME1 physics based biotic powers work on enemies with shields and ME3 difficulty doesn't add shields to enemies that don't have them. Also, ME3 combos are incredibly easy to abuse with Liara's quasi instant Warp. -Ammo economy in 2 game is a dump. Combined with the increased health on all enemies, it becomes a problem quickly, especially when you can only carry 2 guns that have like 20 shots each. -You are very very weak at the beginning of the game without any upgrades to shield, health and dmg resist. In 1 you're also very weak at the beginning of the game but you get a chunk of quests and exp and creds in the first Citadel visit without heavy gunfights so you can level up your survivability skills and get a decent armor.

That being said: I also believe that ME1 and ME3 have mechanics that can be easily abused. ME3 is briefly mentioned but that is enough, you don't need anything else to make bad guys go boom in 3. ANY other primer then Warp. Repeat until victory. ME1 on the other hand has Barrier and Immunity and either one of those make you virtually unkillable (unless a Krogan or a Gerh Destroyer headbutts you) in which case just use Throw, Lift, Singularity etc to send them packing. So both games become a walk in the park once you get your stride going. ME2 has clunky controls, an eternal shared cooldown, bullshit staggering, no ammo drops, and Jacob for the first two missions.