Part of what it was and is, at least in the Showa timeline-Ghidorah was A) the most impressive and visually interesting monster they pulled off, freaking huge and complex, and B) such a badass that it always took more than one monster to bring him down, if they even could.
Remember, you could count Monster Zero as him winning the fight in the end against Rodan and Godzilla alone- he was the only one to get up and walk away afterwards.
Then there's the big hubbabaloo of Destroy All Monsters, which gets way over stated by fans, but it still took about three or so monsters to put Ghidorah down.
After DAM Ghidorah only appeared once more, in Zilla's stories anyways- Gigan, where his suit was falling apart and they couldn't do much with him, so he didn't do much.
He did get an amazing showing against Zone Fighter though, for what it's worth.
Then in the 90s he got out and out beat by Godzilla, if only because of help from the humans, then he appeared in GMK and was so watered down that he had to KEEP getting power ups and he still got murdered, and finally Kaiser Ghidorah just kinda comes out of no where and does so very little.
The ROM trilogy kinda gives him back some luster, as Death Ghidorah murders Momma Mothra and Grand Ghidorah is a straight up badass, but then that was the ROM trilogy and Ghiddy was the only good thing about it.
So yeah. It's just been a series of misuse and overuse over the years.