I think it's a few things:
1) Fitzgerald is the safety valve/chain-moving receiver, and will tend towards always getting the lions share of targets.
2) Michael Floyd got healthy and it was clear that Arizona really, really wanted to get him going. First few weeks it looked like they were forcing him the ball when he wasn't even open. Against Seattle, you can see why they wanted him comfortable because he's really really good.
3) John Brown is running fine, hard to tell whether it's full speed or a 8/10 level, but based on his practice and usage, there's a tingle in his hammies and the team wants to protect him by not overplaying him. Hamstrings are a common injury to re-aggravate, so with Floyd balling out they might as well not play him every snap. He did play as the clear 3rd WR on the field against Seattle when they went 3 wide, so he's clearly not broken. In those spots I saw him frequently lined up in the x receiver spot against Sherman, but he didn't get targeted much in that matchup. And whether it was in the middle against the big fast safeties or on the right against Sherman I think Arizona felt more confident about Fitz and Floyd's ball skills and toughness after the catch, whereas Brown relies more on his speed. With injured hamstrings, Brown's 9/10 speed maybe just wasn't good enough to overcome his the size mismatch enough for the Cardinals to feel confident about him.
Beyond health, I don't think there's any deeper problem. Seattle was probably the worst matchup for Brown due to their size and speed in the secondary. I'd watch Brown's practice participation. I think as soon as he returns to full practice, we can assume he's fully healthy and confident that he won't reinjure his hamstrings. I wouldn't worry about his targets. With this Arians/Palmer deep shot offense I think 5-6 targets is all he needs to have decent games with big upside, and Floyd/Fitz playing well just raises everyone's ceiling with the way they'll run up the score.
Not sure I actually added any info to what we know there, but I said some stuff.