The answer is simple: both the Left and Islam are anti-capitalist at their core.
How is it possible, however, that Islam is anti-Capitalist? Mohammed himself was a merchant by his original profession. Koran never banned trade and private property. Where's the problem?
The answer is in history. Mohammed lived in a simple society of shepherds, craftsmen and merchants. He simply could not have anticipated the modern world with corporations, banks and financial markets. Mohamed banned interest, because usury was a major problem in his civilisation. He just didn't envision the importance of credit in forming the big business.
“Islamic rules governing business partnerships were created between the seventh and tenth centuries,” writes Alan Beattie in his excellent book False Economy.
“They drew mainly from the customs and practices already established in the countries that came under Muslim rule: there is precious little in Koran that determines how partnerships should be organized. (…) “
“One rule was that all payments and principal had to be in cash in a single currency. The goods being traded could not be used to settle accounts. Another was that all partnerships were automatically dissolved on the death of a partner. These laws intersected unhelpfully with the Islamic rules on inheritance, which were laid out clearly in the Koran, and decreed that at least two thirds of the estate of the deceased was to be split between individual members of the extended family. While they may have made Islamic societies more equal, the inheritance rules also made it hard to create and sustain any large-scale business partnership. The death of a single partner meant the partnership was broken up and each of the many inheritors could demand their share in cash.”
This is Alan Beattie's brilliant explanation why the Islamic civilisation was still stuck between the 7th and 10th centuries while the Western world invented banks, joint-stock companies and bourses. Modern capitalism, in short.
The first modern corporation was chartered in 1602. It was the United East India Company or Vereenigde Nederlandsche Geoctroyeerde Oostindische Compagnie in Dutch. “Subscription to the Company was open to all residents of the United Provinces and the charter set no upper limit on how much might be raised. Merchants, artisans and even servants rushed to acquire shares; in Amsterdam alone there were 1,143 subscribers,” writes Niall Fergusson in The Ascent of Money.
It was the financial infrastructure that gave the Western civilisation the edge over the Muslim civilisation. The edge is still there to the immense frustration of Muslims. They consider it as unfair that "infidels"—as they call us—enjoy better living standards, better safety, better healthcare, etc. (We know why: we don't live in the mediæval era anymore.)
Meanwhile, in the West, anti-capitalist movements and political parties of all colours spare no efforts to undo the ongoing Western supremacy. The anti-capitalist ideology goes very well together with pro-Islam, anti-Israel and anti-inequality attitudes.
The Islamic world is not only underdeveloped beyond repair—it is outright anti-modern. It does not want to develop itself, and it shows. If there were no oil and gas in some Islamic countries, the whole Islamic civilisation would be totally irrelevant. Our problem is that the Left wants us to be similarly backward. This is why the Left loves Islam.