Well Lancashire didn't bid because of redevelopment, so that leaves either Headingley, which had an Ashes test last year and gets an ODI in 2011, or Chester-le-Street which has been given several chances to host test matches but, for whatever reason, consistently draws mediocre crowds, and has been given two ODIs in 2011 anyway.
Also whilst Trent Bridge is about halfway up the country geographically, I'd imagine going by population concentration it is actually quite far north (i.e. most of the country live south of Nottingham even though it is in the midlands).
The fact is that England (and Wales) now have more international standard grounds than there will be test matches in a summer - Lord's, Oval, Edgebaston, Trent Bridge, Headingley, Chester-le-Street, Old Trafford and now Sofia Gardens and the Rose Bowl. As long as Lord's gets two per summer (which is disputable, but considering how profitable matches are there and the prestige of touring teams to play there I can't see it changing) then three grounds will miss out every summer. It's just a fact and there's no way around it really.