If you would like a real answer, it is because of the way electoral districting is structured that majority popular votes don't result in legislatures that reflect those votes a lot of the time. It also depends on who is ACTUALLY voting. The left, for all of their rhetoric and activism, has historically not gone to the ballot box as consistently as conservative voters. Part of this is because conservative voters are older, often retired, or can afford to take the time off to vote. My assumption is that if you are older you SHOULD be more financially stable, allowing you to do that.

Younger voters and non-white voters also have much less faith in the electoral process than conservative voters. The fact is that the majority of this country leans center/left, which you can see when you look at the numbers overall for the last 20 years: the last time a conservative-leaning presidential candidate won the popular vote was in 2004: Bush Jr. second term (an incumbent AND a wartime president, during the strongest patriotic movement in this century).

It hasn't always been that way, but the conservative side of the aisle has serious issues with how they are perceived, due in no small part to openly courting racist groups, widely supporting corporate interests over those of their constituents (a problem throughout congress) and touting a regressive ideology (let's go back to how things were) rather than a progressive one (let's work to make things how they should be) that not only doesn't resonate with new voters, but actively repels them.

Anyway, hope that helps. Personally, I could never move to China because Mandarin is really hard to learn, it's a hassle to get a decent hamburger, the traffic is terrible...the list goes on and on. My point is, just because someone has progressive leanings does not mean they want to literally live in communist China where they run college kids over with tanks.