Because the specific stats that armour gives is only useful in niche situations. For example the charred hunter set is good at Laurences fight because of the fire resistance, it may be kind of useful in the dungeons because there is an increase in fire using enimies (like the watchdog and KOTOL) but it doesn't really make or break the fights.

Sure Djura's set has a high frenzy resist it is good against the winter lanterns but you can achieve almost the same thing by spending your insight.

Like the other person said, runes also do a much better job at raising your resistance to niche elements than armour.

So unless your going into a fight where you need every bit of resistance it doesnt really matter what your wearing.