It is because those games were good for their age, changed the genre, and had a lot of "fun" over the top space western moments. Without key "moments" to be memorable or awesome levels, games aren't memorable. On top of that, those two games stick out, especially among the games of their era, and they had a better sense of... "fun" (?).
Assault really didn't. The Arwing gameplay was solid, sure, and the scope was bigger, but it didn't feel bigger, and the game itself was just a confused jumbled mess. It should have relied on the Arwing play (almost) exclusively, it should have had a bigger scope, and it should have either gone totally cheesy or totally ballsy real, but sadly, it took neither.
And yes, the on foot combat was awfully stale. To play Resident Evil 4, GTA: San Andreas, Splinter Cell: Pandora Tomorrow, or MGS3 and compare the combat and controls in those games to Assault's third person on foot combat. It is laughable to call it good compared to those games.
Basically, the game wasn't a unique snowflake. It was just another game released in 2004 and 2005 that failed to differentiate itself, which is why it has largely been swept under the rug, and why we probably won't get even a stage from it in Smash Brothers.