The answer is actually very short, simple, and straightforward: in fact there isn't all that much Satanist stuff in heavy metal, although it may seem so to the casual observer. But since such an answer is hardly satisfying, and would make for a very short chapter, let's modify the question a bit: “Why all that Satan stuff in heavy metal?” Now we’re getting somewhere. While outright “Satanist stuff” is not exactly prominent in Heavy Metal overall, there is no denying that Heavy Metal is deeply infused with references to Satan, demons, and occult lore of all sorts. However, those references take on many different forms, ranging from the overtly Satanic or occult by professedly occult and/or Satanic bands to the anti-Satanic and anti-occult by Christian metal bands, with most instances being far from either end of the spectrum.
Nonetheless, Heavy Metal is often regarded as inherently Satanic. There is no lack of materials on Youtube that decry the purported Satanic or occult nature of other genres of popular music, but in that case the focus is commonly on specific major artists. When it comes to Heavy Metal, it is more often indiscriminately denounced as a whole: meaning everyone and everything that can be in the least bit associated with Heavy Metal – artists, fans, and scenic institutions alike, with no regard for the diversity of its myriad subgenres. This has been the case pretty much since the genre emerged in the late 1960s and early 1970s; but its precursors, blues and rock, were likewise condemned as the Devil's music and faced very similar accusations of engaging with the immoral, the Satanic, and the occult earlier in the twentieth century. For example, a supposed Faustian pact with the Devil has been posited both as an explanation for the extraordinary talent of legendary blues artist Robert Johnson (1911-1938) and as the cause of his untimely death. While the accusations are greatly exaggerated, and many are clearly born from the sordid imaginations of various moral crusaders, not all of them can be outright dismissed, and artists and fans cannot be held entirely blameless for the sinister reputation of Heavy Metal. No other musical genre is as steeped in references to the Devil and the occult as blues, rock, and Heavy Metal, in ascending order.