In Dark Souls 1, pyromancies were special because they scaled with nothing but the flat magadj of your flame, and had no stat requirements. This meant that you would put souls into leveling your flame and purchasing spells, rather than leveling yourself. Now, this meant that it was really really broken if you twinked yourself with a +5 Ascended Flame on a SL1 character and used spells like Black Flame. Additionally, fire weapons had no scaling, unless they were Chaos, which had Humanity scaling, and these weapons were OP for the same reasons.
From decided to "fix" this in Dark Souls 2. Fire damage then scaled off of Int AND Faith. This means that the base damage took a big dive, and you needed to put points into Int/Faith for good damage. However, pyromancies still had no stat requirements, meaning you could use them with 0 faith/int, and do poor damage, or perhaps with high faith OR high int, as in with a sorcery or miracle build, and they would do well. This is the difference from Hexes, which scaled with Int OR Faith, whichever was lower. That meant to do well with Dark damage, you needed BOTH Int+Faith. This was a good system though Soul Memory was supposed to do the same thing, but pyromancies were still the most common form of spellcasting, since they still had high versatility.
And then there is DS3, where they now have stat requirements. This means that to use advanced pyromancies, you need high faith AND int, rather than one or the other, or none at all. There are a few spells that have only int or only faith requirements, but the vast majority will fit in only to a Dark build, where you level Int AND Faith. Admittedly, most of the requirements are low, but they exist. This means that you can't just grab them and plop them into your Str or Dex pure build. You need to put up to 25 in each stat to use the best ones. Only Iron Flesh, Great Chaos Fireball, and Chaos Storm have no requirements, but if you use them with no Int/Faith, they will be less effective (excluding Iron Flesh).
Tell me, When was the last time you were fighting a Strength/Dex Pyro? Last time I did was Dark Souls 2, but it was the most common PvP/PvE archetype in DS1.