Wanting to make sure that the music and voice cast remained the same is a pretty reasonable reason honestly if we learned from D.Grayman Hollow changing the voice cast can really sour the reception of fandom especially when the majority of them are still active hell the Hoshin Engi remake is getting ridiculed on twitter for its new cast as well and it hasn't even aired yet.
If an author cares enough about their creation they can outright refuse a studio to work on it and granted if his reason truly was to give Shuiesha the middle finger he fucked himself up as well because Shaman King isn't a hot property like it used to be with studios clamoring to adapt it so now he had to work to make sure this new series he's working on would be successful enough to get an anime adaptation.
Not really a reasonable reason considering how much time has passed since the original anime and how much the voice acting industry changes over a 15 year time span, especially since some people retired from voice acting and I think someone even died. The author of D-Gray man still accepted the new anime for D-Gray man because she knew it would only benefit her. Why would the author of a property that is not so hot any longer care about sour reception when the anime was going to become a thing and directly promote their manga. Bad reception or not for whatever various reasons, it's still a very rare chance to get a freaking remake, anime costs millions of dollars to be made and you bet a Shaman King remake would've been pretty pricey if they gave it at least 3 cours and were set to do things properly this time.
Edit: Also, the majority of the people that would've watched the remake wouldn't have cared about the original voice actors in the first place because the majority of the people watching it would've surely been newly introduced to the series anyway.
Yeah, an author can refuse something like that, especially if he has a new very popular series that is ongoing and he thinks he can get a better studio to work on it or if he wants his favorite studio to work on it when they are free.
But we're talking about a remake here, of a manga that is long past its prime in the quickly moving manga world. I really do feel like he refused the anime because he wanted to sever his ties with Shueisha completely, especially after the rights transfer of the franchise to Kodansha has come up, it just seems like the core reason to me. There has been a trend lately where older manga from the 80's or 90's have been getting remade, but most of the series that are getting remade were absolute titans in the manga world. Then you also have stuff like FMA and HxH which already had successful adaptations, but were open for improvement with new adaptations that would adapt more contents of the manga. If you look at Ushio to Tora, you might say, it isn't really a titan, but for Weekly Shounen Sunday it is, it still remains one of their biggest properties to date after they've fallen behind and it never got a decent anime. Shaman king is from WSJ, which has a lot of other series that are more popular and successful that could get remakes or proper adaptations for the first time, the fact that Shueisha wanted to go for Shaman King over something twice as big like Kenshin for example, shows they were determined to bring back the series the right way.