Why was Ezri Dax the polar opposite of Jadzia Dax?

Well, remember that Trill hosts being super different was part of Dax's whole backstory - it was a pretty huge deal. Curzon Dax was very different from Jadzia Dax. And I'm pretty sure that Jadzia also wasn't a serial-killing psychopath, like Joran Dax. Plus, it was pretty well established in Star Trek: The Next Generation that in a situation like Jadzia's/Dax's - in which the host dies suddenly - that finding a replacement host is pretty much an emergency, and that it's not necessarily easy to transfer hosts. (The 1991 episode "The Host" is pretty much entirely about that.)

So it would've made sense for Dax's next host (Ezri) to pretty much be the nearest Trill that could be found to minimize the wait for a Trill host (the Next Generation episode establishes that a non-Trill host can only take the symbiont for so long). (As Izkata noted in the comments, Ezri states as much at one point.) Given that and the length, rigor, and selectivity of the training for humanoid Trill to become hosts, it made a lot of sense that the nearest available Trill for said emergency wasn't necessarily fully ready to become a host. (Hence a huge part of the awkwardness that seems to contrast with Jadzia - Jadzia had tons of prep for this, while Ezri's getting used to something that she was thrown into without the slightest bit of preparation.)

In short, given the nature of the show and the way that they'd portrayed Dax's history - not to mention Trill culture and biology - it's pretty logical to have the new host be her own person, which likely influenced how the writers decided to portray Ezri. I haven't found any specific non-plot-related reason for the personality change, but it made sense within the general Star Trek universe in general, and particularly within DS9 itself. (Plus, it made it more interesting than just making a carbon copy of Jadzia, in my humble opinion, but I don't have substantial evidence that any of the producers were thinking along those lines.)