Here's a good sum-up of NJPW's dark ages in the 2000s from reddit. https://www.reddit.com/r/SquaredCirc..._ages_of_njpw/

Now, here's my bad write-up. A little background, for those that do not know, in the 90s, Japanese wrestling, puroresu, would be at an all-time high hotspot in Japan and the leading promotions were AJPW (headlined by the Four Pillars of Heaven selling out Budokan Hall almost every month of the year and drawing a million dollars in the gate) and NJPW (headlined by the Three Musketeers: The Great Muta, Masahiro Chono, and Shinya Hashimoto where NJPW would legit sellout the January 4th Tokyo Dome shows, drawing 50k+ people as opposed to around 25k people that the modern NJPW are doing now, which is still good than what happens in the 2000s, but more on that later. At one point in the 90s, I really believe that NJPW and AJPW were better than WWF/WCW in terms of match quality and making money. So what happened in the 2000s?

Well, in the late 90s and 2000s, to my recollection, MMA in Japan was really getting a strong footing and NJPW founder Antonio Inoki really has a big boner when it comes to that stuff when it comes to legit fighting and proving that pro wrestlers, preferably his wrestlers, were the best shoot fighters. So he decided to send Yuji Nagata, some might recall that name when he wrestled in WCW, who had a strong amateur wrestling background, but hasn't really trained for that in years to have an MMA fight with Fedor Emelianenko and Mirko Cro Cop. Nagata would lose both fights in about a minute in the first round, effectively ruining his credibility and drawing power during his prime years in NJPW.

So Inoki decided to then actually bring MMA fighters to NJPW and put them over the actual wrestlers. He also decided to push Shinsuke Nakamura at the age of 23 to win the IWGP Heavyweight Champion, becoming the youngest to do so and this was a decade before he became Swagsuke that we all know and love. He was more of a dull wrestler at the time with a strong shoot background and MMA record with no charisma/personality (shocking, I know). Another thing that really devalued the IWGP Heavyweight Championship was having Bob Sapp (one of the biggest names in MMA in Japan at the time) win the title. Now Sapp winning the title wasn't bad, per se. I mean putting a big name in Japan at the time would raise credibility to a company. He had this video for crying out loud showing how big of a deal he is.



So, why did the IWGP heavyweight championship get devalued? Well, Sapp lost an MMA fight while being the IWGP Champion. So then, he vacated the title after he lost the fight thus partially ruining the IWGP title.

The NJPW crowds did not really like the idea of bringing MMA fighters (since they weren't really entertaining wrestlers) and didn't like those fighters going over guys like Yuji Nagata and Hiroyoshi Tenzan. So the crowds started dwindling to the point where the 01/04 Tokyo Dome shows were at an all-time low. The lowest crowd it had was 18,000 people in 2007 and maybe 18,000 came to the show, but I doubt all 18,000 paid for the show showing how bad NJPW was in the 2000s. It was actually named Worst Wrestling Promotion by WON in 2004 and 2005 (compared that to where they were named best promotion from 2012-onwards showing how much traction they've been getting).

It came to that people were so sick of Inoki's boner for putting wrestlers in MMA fights and losing those fights and losing their drawing power and pushing MMA wrestlers to the moon (while being not so great wrestlers) and he lost his grasp in ownership of NJPW and was removed from power in I think 2007 or 2008. It's like if Brawl-for-all was a longer-lasting thing and actually ruined a prominent wrestling promotion.

Eventually, with new ownership and some timing and some luck and a new booker named Gedo and a tremendous wrestler name Hiroshi Tanahashi that wants to change the perception of NJPW at the time and making some good pushes to promising wrestlers, NJPW rebounded themselves to being the clear-cut #2 wrestling promotion in the world as we know today.

TL;DR. I will say LOL Inoki for pushing MMA into a wrestling promotion and backfiring in the worst way possible, ruining a promising promotion.