I'm not saying don't audition people and try to find the best He-Man you can get. But just because an actor has blonde hair and acting talent doesn't mean he'll be a good casting choice for the role either. I mean I'm 6' tall with long blonde hair and blue eyes. I have experience in both film, television, as well as stage acting. Does that mean I would make a good He-Man? Heck no! Why? Because I also happen to be about 50 lbs overweight. I would have to turn that 50 lbs of fat into 50 lbs of muscle in order to even have a chance of qualifying for the role. I Just don't see Skarsgård or Wakefield being able to bulk up enough to properly look the part.
I'll give you another real life example. I'm trying to produce a trailer for my live action Thundarr film to help develop interest in getting the full length feature produced. There has been plenty of local actors interested in playing Thundarr, Ariel, Sabian, Zogarr, and the Pig-Mutants. But only three people asked to audition for the role of Ookla. Unfortunately the one who was most qualified for the role was disqualified because he's not local and is way above my budget for the trailer (although I'd love to cast him in the actual feature). He was Mark "Conan" Stevens, who is probably best known as "The Mountain" Ser Gregor on Game of Thrones. The other two are local boys trying to break into the film business. Neither really has much of a resume. One of them, a guy named David, stands 6'6", weighs about 240 lbs, and is very physically fit. While not quite 7' tall, there are little tricks that you can do to make him appear bigger on film (thick soled shoes, lifts, camera angles, forced perspective shots, etc). The other guy is 6'0", and weighs about 150 lbs. Between those two, who do you think I'm gonna end up casting?
Of course casting someone as a mutant cat person whose dialogue is going to be replaced in post with animal noises and casting someone as an incredibly muscled superhero in a fur loincloth aren't exactly the same thing. Acting talent and charisma aren't nearly as important when you're wearing a ton of prosthetic make up and have no dialogue (at least none that the audience will understand). However you still will want someone with more or less the same physical characteristics as the character they're auditioning for.
Dwayne "The Rock" Johnson, for example, is very physically fit, quite large at about 6'6", incredibly charismatic, and not a terrible actor. But would he make a good He-Man? No. Why? Because he's half black and half Samoan, while He-Man is white with blonde hair and blue eyes. He might be well suited to a secondary character, perhaps one that's not necessarily one ethnicity or another (Man-E-Faces, Mekaneck, Buzz Off, etc), but for He-Man he'd be all wrong. Skarsgård and Wakefield have a similar, though different problem. Their acting ability isn't in question, nor is their ethnicity, but rather their physical presence. Physically, I don't think either of them can fit the bill.