Why not just port the Mass Effect LE multiplayer as is without the remaster?

As far as your question concerning the ORIGINAL Multiplayer not being included, probably because it would feel "out of place" with the new graphical upgrades to the single-player (not that most fans would probably care, but it was an aesthetic choice that the developer has every right to make).

I know that they originally did not include the Multiplayer because they were not able to upgrade the graphics on the single-player AND multi-player modes and have it done in time (perhaps too ambitious, perhaps EA time schedule? I don't know). Maybe it is wishful thinking but I think that it would be smart to do the initial launch of Mass Effect Legendary Edition on May 14th, then a few months later (after they have had the proper time to work on it while juggling other projects), release the Mass Effect 3 multiplayer (with the enhanced graphics) as a free DLC. Maybe not though, since they are modifying the war assets on Mass Effect 3 on the Mass Effect: Legendary Edition to function (not decreasing the total amount of Readiness with the modifier) to work without playing multiplayer. They could also be pushing people towards the Mass Effect: Andromeda multi-player, I don't know. As fun as the Mass Effect 3 Multi-player is, I still play the Mass Effect: Andromeda multi-player quite a bit still to this day (mostly because I have Mass Effect 3 on Xbox 360 and my Gold subscription has expired and I have Andromeda on PC).