Why Mazinger-Z or Great Mazinger never were used against the Vegans? They showed up in the Go Nagai manga and in two Crossover movies, but never in the anime series. The remainder time Kouji limited himself to pilot a tiny flying saucer or a Mid-Season Upgrade instead of actively helping Duke by using his Humongous Mecha to battle. And Tetsuya only showed up in one story of the manga. I get Duke and Grendizer were the stars of the show and the anime-makers surely thought they would become less prominent if the former pilots and mechas returning. Yet, why ignoring so blatantly and completely the past series?
Well according to the stats handed out by Nagai Mazinger has around 20.000 hp Great Mazinger about 30.000 while Grendizer has 300.000 hp... So it seems the older two are SEVERELY outclassed.(Obviously this is can lead to other headscratcher]
Well, a couple of problems and contradictions in here. In the Great Mazinger vs Grendizer movie, the first was powerful enough to keep a good fight against Grendizer (in fact the Vegans stolen it from the peace museum where it was stored since they discovered it was the previous most powerful mecha in Earth, and if Kouji didn't arrived at time to tell Duke which was Great Mazinger weak spot, he was going to be defeated), but of course, you can consider movies as not-canon. Second, the anime is according to Go Nagai himself not-canon also, since he had many problems with Toei and Shingo Araki about how to adapt his ideas into the animation (this explain the SOOOO many plot holes, inconsistencies, and lack of past characters in the Grendizer anime) Third and last, about the Go Nagai data about Grendizer HP...it has been a true headache, since old Nagai seems to be stubborn about loving and insisting about Grendizer being his most powerful robot ever created (yeah, even more Mazinkaiser...go figure), he thinks Grendizer is indestructible or something. But there are a couple of truths that shows contradictions in this. First you can get the famous battle in the anime (yeah, i already know i said Go himself consider it non-canon, but keep reading) against King Gori, this monster was able to rip easily off an arm of Grendizer in one grab (and also Grendizer NEVER was able to really defeat him...they had to shut down his controller device) Also it was stated that Grendizer suffered many damages along both series (manga and anime, so this IS cannon), and since the secret behind his Gren alloy was way beyond the compression of humans, they just "patched" every fissure and hole after every fight he had with the new alloy z of Great Mazinger. Also, in the original manga (written an draw by Go himself), when both Mazinger and Great Mazinger pilots was brainwashed and attacked the city, Grendizer got a heavy time fighting against both of them at the same time...and not because he didn't want to harm Kouji and Tetsuya. Another one, the Super Robot Wars franchise, although not canonical also, Nagai and his team has been involved very deeply in most of the games of the saga, if you compare and extrapolate stats, Grendizer is in all games he appeared just a "little" better than Great Mazinger, while Mazinkaiser is always one of the most powerful robots available and have way better stats than Great Mazinger (we can't just compare Grendizer vs Mazinkaiser in one same game, since Nagai cowardly refused to accept to have both of them in one simple of them...in fact is a well know clause Dynamic studios put in every SRW with their mechas in the roster) As a final touch, and with Nagai own pen, in the electronic manga "Nagai Go Manga Gaiden - Dynamic Heroes", Mazinger fights Grendizer...and the first wins...It seems also Nagai sometimes forget he made most of his money (although he is a legendary mangaka and has created many other non-mecha series, before and along all his long career) from Mazinger and not Grendizer, and not pleasing the fans is a veryyyy bad idea (EDIT: in some interviews in 2012 and 2013, he seems to withdraws about his decisions, and now he is putting Mazinger in the front line as his favorite and most powerful robot...it was about time old Go...)
Where was Shiro as Kouji remained in the Space Science Laboratory? This is another trouble born of the anime-makers disregarding nearly all ties with the former series of the trilogy: Shiro never showed up and Kouji never mentioned him, so we do not know why Kouji was not taking care of his thirteen-years-old little brother like he did in the original series, we do not know WHO was taking care of him (likely candidates are Tetsuya and Jun, or Prof. Yumi), and it did Kouji seemed a jerk.
This is an anime problem only (in the manga was different), say thanks to Toei's Executive Meddling problems with Go Nagai, which ends making an anime with a LOT of plot holes, so much that most Mazinger fans don't even consider Grendizer as a part of the series (they use now Mazinkaiser as the third saga), and the Grendizer fans considering their anime as a self-contained and apart universe...