Maybe they want a crazy amount of money and promoters just aren't willing to pay it whereas festivals are. MLK obviously gives LP huuuge money since they sell out the shows all the time.
Yes, having big successful shows definitely correlates to LP coming back. Milano was MASSIVE and look, LP returns. Expect to see Italy in the next world tour too. Russia might be the best example. 2007 was so big for them and they've returned in 2009, 2011, 2012, 2014. Poland is another good example. Wroclaw's show was huge after Orange Warsaw 2012.
I agree with Jonas. Looks like they'll take July off, which sucks, but Chester will need a break. There's like a week or so between the STP last show of the tour and LP at Rock In Rio....that pretty much guarantees they'll play the same stuff for the rest of the touring cycle or at least May and June. Maybe STP finishes the CD in July and/or tours a little bit with a few shows. After Europe it's STP full on, probably, which is why the North American Tour wasn't made up. Maybe Fort Minor returns then.
May and June surely will get some more shows. What sucks is that they probably won't do a full USA tour there. Why not play from May 9th until June 30th and headline shows in between all of the festivals? Make up some canceled shows, play Texas, California, Florida, northeast, etc. Hell, play Mexico. That'd sit well with the USA/Canada fans since the whole tour was canceled. I see a lot of people on the LPU FB page bitching about European shows being announced, lol.
They're only playing on weekends right now in May and June. Weirdest thing I've seen in a while.