There's two parts to this question. For both, the reason largely comes down to “because the dominant instrument in metal is electric guitar“.
Why does metal have more muffled treble than some other genres?
It might not be one's first though to describe electric guitar as muffled, especially not distorted lead guitars. But actually they are, in a sense: no matter what amp / effects setting you use, the last thing this the signal passes through is the cabinet. And guitar cabinets only have one kind of speaker: a low/mid driver. Such a speaker strongly rolls of the treble above something like 6 kHz, which is why even a brutally scooped / distorted metal guitar actually has less high treble than an acoustic guitar. (This frequency response is needed to make the sound enjoyable despite the brutal clipping transients introduced by the distortion.)
The other instruments don't contribute much treble clarity either: in metal, singers focus on producing loud tone rather than breathy / consonant-rich lyrics. Metal bass tends to sit rather far back in the mix (and often really is pretty muffled). The by far strongest treble contribution comes from the cymbals, but ironically, it is because metal drummers play so loud that this doesn't add much clarity either: loudly played cymbals ring out as more of a white-noise carpet rather than crisp transients as you get from delicate play on the hi-hat or drum machines. Anyways, engineers in metal put much more emphasis on the drums than cymbals, and these don't offer as much treble as lower frequencies.
Why does metal not have as low notes as some other genres?
For one thing, metal is supposed to be in-your-face loud, but low frequencies require much more energy to be perceived as loud than mid/high frequencies do. Especially on records with a limited dynamic range (thanks, loudness wars), you simply don't have the headroom to put in the required bass-energy to compete with the loud guitar and snare drum mids. Mind, on metal concerts there is usually a pretty massive amount of bass, just your ears are then so numbed by the ridiculously loud midrange that you pretty much only feel the bass with your body anymore.
But also, bass frequencies aren't very useful for definition. The loudest bass instrument in rock is the kick drum, but most metal drummers want a rather “clicky” kick sound so that double bass assaults will come out with good rhythmic definition. Try that with a thumping hip-hop bass-drum sound, any you'll end up with pretty much just low-frequency rumble, not distinguishable 16th notes.
The actual bass is, again, either just so far back in the mix that it's hardly perceived, or it focuses on the midrange – for, again, better definition of fast passages, and in order to be properly heard against the guitars, which in metal often go pretty deep (but even an eight-string djent guitar has so much midrange that it's not perceived as so deep as synth sounds in electronic genres).