It's thanks to internet-driven user-generated content, her striking design, and the viral spread of early music on Nico Nico Douga.
Crypton Future Media marketed her as an approachable canvas rather than just a locked-down corporate product, allowing independent producers to create and share hits freely
Basically ... internet is the one who made her famous.
It is admirable in a way, given lots of Japanese marketed content rarely get as popular as Miku internationally.
Sonic was briefly more recognizable among children in the United States than Mickey Mouse during the early 1990s (specifically around 1992–1993), according to historical market research and Q Scores.
That's his potential.
Sadly he could not maintain his popularity in mid and late 1990s.

Yeah! The '90s was pretty much when Sega peaked! They tried to make a mascot better than Nintendo, and they succeeded! They released the Mega Drive solely to compete with the NES, and Nintendo brought the SNES to the table, but the Mega Drive still held its own! And because of that, Sega became just as much of a corporate titan as Nintendo was at the time. The "90's Console Wars" between Sega and Nintendo were still famous to this day. Sega of Japan may not had approved of Sega of America's marketing methods, but the high risk - high reward worked for Sega well during the '90s!
But if your kids had any of their young friends over and you told them "Sega were just as much as big as Nintendo was" or "Sonic was as popular as Mickey Mouse or Mario", would they believe you? Granted they would perhaps be too young to know such words as "subsidiary" and "third-party", but while Sega still retain their popularity, being a subsidiary that mostly produce games over making them, they're just not one of the titans as Nintendo, MicroSoft and Sony Entertainment.
