Why Chrome becomes unresponsive while other pages are loading in background tabs?

Because the developers who are working on the browsers have near unlimited resources at their disposal, and for the most part don't care about people with older computers or working in resource- or bandwidth-constrained environments. There's currently (2015...) a huge drive to improve the efficiency of JavaScript, but as yet only a handful of browser extensions that will allow you to disable background tabs. In my opinion, the default on a webpage should be that it has 5 - 10 seconds to render the page, after which JavaScript gets disabled; unless it's an intelligent Web Application. And even if it is, it should get suspended while it's not being displayed, with the exception of if it's playing media, or if you explicitly set it to run in the background, presumably so that it can pop up and notify you about things, or graph something over time, in the background.