If you insist on taking this literally as being about material, tents (which some translate 'succah' as) have historically been made of animal skins, and you can also get fish leather, so Leviathan's skin might not necessarily be unsuitable.
It depends on exactly what kind of animal the Leviathan is understood to be - The Gemara uses 'salted fish' as a comparison for the preserved female, and the rabbis of that era might well have understood whales as giant fish, but honestly the descriptions often sound closer to a dragon - Which can be an umbrella term, so that doesn't exclude it from also being technically a fish, mammal or reptile (the appellation 'Rahab' is sometimes translated as 'crocodile').
Supposedly we will eat its flesh, but some believe the laws of kashrut will no longer apply, so that doesn't narrow it down much (note that actual whale skin is eaten amongst some people, such as the 'muktuk' of Greenland, but whale skin - unlike whale bones - specifically doesn't seem to be used for construction).
As for 'Why not all-loved cows', well quite simply, this is one very large and supposedly luminously splendid skin to divide amongst millions of people for sukkot, head-dresses, covers, necklaces (presumably like a leather thong?) and amulets, depending on the person.
In comparison, it would take a ridiculous number of cows, plus the idea that this one material being good for all those things is probably meant to indicate its WTC-esque superiority. Cowhide wasn't luminous at my last check, and most people understood cowhide tents as the earthly norm.
This is all supposed to take place during the 'Days of Moshiach' so many ideas about it indicate novel experiences and fantastical standards rather than mundane, everyday ones.
Much like how the heavens are imagined to be paved with sapphire rather than with the limestone of Jerusalem painted blue.
If you are concerned about the smell (tanning cow hides also smells bad), well I imagine G-d is supposed to take care of that.
If you would really prefer a cowhide sukkah, maybe ask G-d to make you one from the hide of the Behemoth (which I'm pretty sure is smaller than Leviathan) instead, whose meat we will also supposedly feast on? His skin doesn't seem to have a designated use yet (although like whale skin, cow skin can be made edible).
No word on whether we get to eat/utilise the Ziz either...