pro-business party like PO forming a coalition with a socially conservative, populist, pro-rural Poland B party like PSL.
Except the PSL aren't Polska B by any stretch of the imagination. The PSL are the rural middle classes - the type that owns profitable farms and so on. The party itself isn't socially conservative, nor is it populist - it's a centrist party that doesn't really have much of an opinion on social issues. As for economics, if it wasn't for the fact that their voter base would be hideously upset with them if they removed KRUS (particularly as a lot of wealthy PSL supporting farmers are paying peanuts under this system) - they would have abolished it a long time ago.
The PSL make such good coalition partners because of their lack of real interest in social issues combined with a centrist political stance. Hence - why would anyone say anything about PO being in coalition with them when it's well established that they are more or less centrist and able to share power with almost anyone?
Now, compare this to that bunch of Marxist thugs that Lepper led, and you soon get the idea why things were different.