It's easier to reduce the capabilities of one job than to buff the capabilities of 20 other jobs. Not only is it more efficient, cost effective, faster, more intuitive to just weaken one job, it also requires less testing. Furthermore, if you buff 20 other jobs you vastly increase the probability of something unbeknownst slip through the cracks and now the playerbase jumps onto the next overpowered melee.
When you buff more than 1 job at a time, it takes months for players to recognize the new damage hierarchy and more time still for the devs to catch on. So lets say they buff WAR DRK THF BLU DNC for more damage in a single patch. People play with them and after a few weeks realize that now DNC and BLU are OP when they spam some WS or JA or spell, then it'll take for SE to catch on and nerf those jobs.
It's easier to nerf than to play catch up for 20 other jobs especially because by buffing other jobs you create a whole new host of potential problems and exploits.