Why? The real question is why not? The playerbase is filled with awful attitudes with no desire to win. That defeats the purpose of ranked, a place where you should always be trying to win no matter the circumstances. Bring it to a server with no real consequences if you troll or afk and you get what? You get PBE ranked. There is literally 0 point in having ranked on the pbe other than for people who want to flaunt a diamond + border for getting less trolls than the opposite team. No it does not show that you're actually diamond. No it does not show you carried your teams to get that border. No it does not show the countless hours of work you put into it. No it does not translate into anything nor does it mean anything.
If it's that bad, why do people play it? That's a good question and I think I can answer it. I started playing ranked on pbe late s2. The atmosphere was PERFECT. It was everything you dreamed it would be like in high elo. A relatively small player base with good mannered players. Since PBE was new, there were VERY few toxic players. Then comes s3, some rivalries are made from last season. People knew eachothers names from playing against them so many times. It was still fun, but later it started to get over flooded with random people. Then comes s4, PBE had pretty much lost that magic touch. Many of the challenger/diamond accounts had been disabled for inactivity. The ranked player base had become a wasteland of trolls.
It is still like this today since s4. It's a place where people can attempt to get the rank they could never achieve on the live server. It's a place where they could try to experience that feeling of high elo. It's also a place where they can do whatever the hell they want to. My overall point is that ranked should be disabled, but keep the normal draft system. People who like the pick/ban system can still have that in normals or customs without having to deal with as many trolls.
As for those you currently are placed in a division or have a border from last season. This post is not insulting you or your rank in a derogatory manner. I'm simply stating your rank doesn't correlate to your actual skill. Only a handful of players in diamond/challenger can say they actually deserve to be where they are. Hopefully removing ranked could actually restore pbe to its old glory.