The marketing was incredible because it leaned heavily into "What is the Matrix?" and didn't spoil the plot. It made the movie itself amazing, because you had no idea what to expect.
I cringe just thinking about how that movie would be marketed today. The trailer would probably start off with all the action scenes voiced over by Morpheus explaining exactly what the Matrix was, followed by Agent Smith monologuing about how humans are a virus that needs to be wiped out.