Until the last 100 years the "left" in Western politics hardly existied.
The people that ran most European countries came from land owners and the "rich". The two parties in the UK years ago were the Whigs and the Tories and were centre or right parties, we had no left party.
The working man had no say in anything that went on and had no rights, no votes, no trade unions.
If you look at the working conditions people had 100 years ago they were awful. Kids as young as 8 working in factories, 12 hour days, no holiday pay (or holiday), no sick pay or redundancy, no social benefits.
The "bosses" screwed the workers into the ground and drove many of them to early graves and became VERY rich in the process.
The left or socialist parties (and trade unions) came out of the desire to give the working man some rights.
We did not get the first Labour (left) government until the 1920s, so only about 90 years ago.
Of course the early labour governments had a lot to do to provide basic support for the working man, like the health service, social benefits, holiday pay, sick pay and so on.
Gradually these things became more acceptable (the Tories were hardly likely to remove them when they came into power).
This gave the "left" less socialist things to fight for, so Tony Blair and his cronies decided the only way to get elected was to move the party more towards the centre.
Cameron now wants to move to Tories away from the right (or far right) towards the centre to make THEM more electable.
So now we have three parties (Labour, Liberal, Tories) all hogging that middle ground with very little to separate them.