Why is UP and Bihar always clubbed together ?

In all discussions across India why does UP and Bihar come up together ? Sure both share a certain set of similarities but so do many other states ... parts of MP are a lot similar to parts of UP, parts of UP are included in NCR, parts of Bihar are similar to WB, culturally UP, Bihar, MP and many other parts on North India are strikingly similar. Economically both are distinct. The only thing that does appear same is perhaps the kind of politics that is prevalent in both the states, but 'cast politics' is becoming more of a norm in India now.
Even the way the native people of UP and Bihar speak is different. The poor people of both these states who migrate to other places may look alike but that is how each and every poor of India looks like !!!
And just like any other state of India UP too have hoards of settlers from Bihar. Perhaps the reason both are clubbed together is that no one in UP has ever closed the doors to the Bihari, be it a student fighting the odds to become an IAS officer or the laborer who struggles hard to earn the daily bread.

PS: Please don't smudge this post with 'hate' comments and take the topic to some place else. I'm aware that both these states are scorned at and they don't deserve it (but even people like Mr. Amitabh Bachan like to make it a matter of joke with his funny accent of a 'chora ganga kinare wala' when he's shown walking down d polio booths, while he chooses to speak in sparkling english when he's speaking about the surge that India has made in various walks of life and this 'chora of ganga' is then walking along the lovely cost line of mumbai; more than being a case of what we want, its a case of what we expect that the media decides to show).