The self-serving Leftist mythology is that it is a product of oppression - but the opposite is the case: Leftism is always a product of increasing wealth and freedom - or indeed of established luxury and status.
The earliest Leftists were the industrial proletariat - who were probably the wealthiest and free-est working class group who existed in the world at that time. Early (middle to late 19th century) socialism became established, therefore, among the well-paid workers in the urban areas and among new industries such as coal mining, shipyards, steel making etc.
For example, the late 19th century miners in Newcastle upon Tyne were so wealthy (for their time) that they were renowned for their fancy clothes and expensive pastimes such as drinking, gambling and having fun. They were, indeed, so well-off that their wives did not need to earn any money - and it became a source of 'macho' pride to be a sufficiently successful bread-winner that the wife would stay at home and look after the house.
Socialist miners formed the backbone or shock-troops of British socialism until the unsuccessful strike of 1984.
Meanwhile, a few miles down the road, the farm workers remained extremely poor, with no money left over for fun and games, and their wives and children needed to work as much as possible simply to get enough to eat.
Yet Leftist parties almost always opposed the Industrial revolution, which - following Marx's mistake/ deliberate error - they depicted as impoverishing the working class. In fact, as Greg Clark shows in A Farewell To Alms, the Industrial Revolution benefited the poor far more than the rich - and ended up by abolishing structural poverty altogether - fought every inch of the way by socialism, which afterwards re-wrote history and took the credit for the improvements.
If you read honest memoirs by the likes of Shelby Steele and Thomas Sowell, it can be seen that an analogous situation applied in the USA. The 'civil rights' era came after the great improvements in the wealth, status and freedom of the ex-slave population in the USA the situation. As usual, Leftism took credit for, and exploited, what had already been achieved without Leftism, and indeed most fought by the most Leftist parties.
The same applies to feminism. Steve Moxon demonstrates (in the Woman Racket) how feminism simply exploited the already achieved increased freedom and wealth of women; and after the hard-fought extension of the franchise to working class men women were simply 'gifted' with votes.
My point here is certainly not to argue that Leftist phenomena, e.g. antiracism and feminism, are indeed the ultimate 'good causes' that they are portrayed to be in the current mass media - but that, good or bad, the genuine successes were not the product of Leftism.
Leftism came after, declared them good things by lying about how things had been comparatively and beforehand, and rewrote the history to claim credit.
Thus Leftism is a phenomenon invariably associated with increasing wealth, freedom and status - because when people really are oppressed, poor and miserable they are too weak - when the laws and social practices really are against them, when they really are 'minorities' - people are much too frightened, vulnerable and exhausted (and with good reason!) to become Leftists and mount political campaigns.
The genuinely oppressed are those you never hear anything about from Leftists.