How am I being ignorant? Explain yourself asinine naive.
First off: DEADLINES. Caps are for emphasis. The Project M team has all the time in the world to do whatever the **** they want to the game. Meanwhile, Sakurai and his team have to release the game on a tight schedule.
Project M has complete freedom when it comes to adding content. Not just thanks to the above time question I elaborated above, but because they aren't bound to a contract nor subject to legal issues.
You know why Project M doesn't have any budget? Because they're not employed by anyone. The budget that goes into games is to pay every party involved into the game's production. You know the Staff Credits list in every game? All those people are paid to make the game. Programmers, designers, animators, voice actors, translators, and so on...
How long as Project M been out and getting updated? Perhaps before Smash 4 was released and still going after Smash 4's release. Once again: TIME. They have TIME Sakurai and his team don't.
Also, this:
Project M is a community game mod project with zero deadlines and complete control over the content they provide.
Smash 4 have a limited amount of development time, no matter if short or long, a long list of goals to meet and expand for the final build, not to mention, they have Masahiro Sakurai as the director. A smart guy with wild ideas.
Also, there are TWO versions of Smash Bros. this generation.
And as you can see from up to now, there is a LOT of content for both versions, even more for Wii U. Obviously, remastering an N64 stage was VERY low on their priority list.
There's a difference between a professional development team bound to contracts, legal issues, time constraints and so on, and a team of amateurs who modify an existing game in their free time.
It's apples and oranges. Both are fruits, but they're different.