If I had to guess, it's probably because of EA's involvement in 2 and 3. You've played all 3, so you know of the same subtle differences between 1 and 2 and 1 and 3. They're the same game, and they're all fantastic, but little things were different between the original and the sequels.
The change from unlimited ammo to "thermal clips" (which is almost exactly opposite what the codex entry in ME1 said about how future weapons work). Dozens of different models of each weapon in ME1, but only having 2 in ME2, back to several different models in 3. The changes to customizing your weapons (practically nonexistant in 2, then brought back in 3 but not as extensively). The loss of the Mako rover (but later a new rover, The Hammerhead, in the DLC for 2, but vastly limited). Not being able to move around the explorable areas (Citadel/Presidium/Wards, Noveria, etc) as extensively in 2 as you could in 1. It all reeks of EA branded projects.
I'm not a big fan of EA games, because it seems like they always rush out projects instead of doing proper quality testing and programming. For example, to me the graphics in 1 were much smoother and more polished, wheras 2 just seemed to have a rougher edge to them. Maybe that's just me, though...
Yes, i've played through all three of them and all of the DLC that was offered, i'm tempted to go back and play the original because I haven't played it since it's original release and my memories of it are a tad fuzzy. Even though it conflicts with the first game, I welcomed the addition of limited ammunition opposed to infinite, I like being under the pressure when I run out of ammo that I have to switch to a sidearm. I do very much agree that the last two did feel more linear, in story and exploration. I do need to note that The Shadow Broker DLC for ME2 is probably the best DLC I have bought besides Bioshock 2 Minerva's Den, it was excellent and rounded off the game for me, and actually carried over into ME3.
I'm also not much of a fan of how EA likes to squeeze money out of their games, when you play online the more you play the more credits you earn. You eventually spend these credits on equipment packs that unlocks random weapons or characters, the better the case the better value of equipment. So how it's set up, you can play through about 5 or 6 Bronze levels in about two hours and have enough credits to purchase a 60,000 credit specter pack OR you can spend 240 MS points or about $3.00 to buy a pack. Being it's random, unlocking characters and guns can become very aggrivating. Needless to say tons of people are buying into this, a friend I play with dropped $40 into packs trying to unlock one character to no avail. It's money grubbing as EA does best, the unlock system as a whole feels like they built it to annoy you and make you want to spend real money on packs.
Definitely post up your thoughts once you finish ME3, i've been playing it since it's been released and still really into it.
I have to agree with Jedi. EA kills everything!!!! Look at Dragon Age 2!
Anyways, I love everything about ME3 except the ending of the campaign which truly is as bad as they it is.
The Multiplayer however is really fun and can be challenging at the silver and gold level especially if you aren't working together with your teammates.
Me and a friend of mine can tackle Silver pretty easily now, but still haven't completed a mission on gold, although we did get to wave 10 a few times.
I'm still waiting for a Dragon Age game to be good, I preferred DA2 over the first one but still really didn't enjoy either all too much.