The OSI makes their position quite clear in the annotated Open Source Definition:
6. No Discrimination Against Fields of Endeavor
The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
Rationale: The major intention of this clause is to prohibit license traps that prevent open source from being used commercially. We want commercial users to join our community, not feel excluded from it.
From an open-source utility perspective, this makes sense. Neither GNU/Linux nor the Apache web server (to name some seminal FLOSS successes) would have taken off long-term if they didn't allow commercial use and redistribution, being commonly resold (modified or commercially bundled with other software) or run within commercial services.
On the free software side, freedom is paramount, so any terms that dictate how you may not redistribute the software would not be accepted without extremely good reason.
Considering that the modern ethos of FLOSS was formed by Richard Stallman's formation of the GNU project, it is worth looking at what values he put forward in the GNU Manifesto regarding commercialization.
An awareness of service-based business models:
“Nobody will use it if it is free, because that means they can't rely on any support.”
“You have to charge for the program to pay for providing the support.”
If people would rather pay for GNU plus service than get GNU free without service, a company to provide just service to people who have obtained GNU free ought to be profitable.(4)
If your business needs to be able to rely on support, the only way is to have all the necessary sources and tools. Then you can hire any available person to fix your problem; you are not at the mercy of any individual. With Unix, the price of sources puts this out of consideration for most businesses. With GNU this will be easy. It is still possible for there to be no available competent person, but this problem cannot be blamed on distribution arrangements. GNU does not eliminate all the world's problems, only some of them. [...]
Footnote 4: Several such companies now exist.
A paradigm that prioritizes the economic value of using and modifying software and dislike for an economic model that relies on a copyright monopoly:
“My company needs a proprietary operating system to get a competitive edge.”
GNU will remove operating system software from the realm of competition. You will not be able to get an edge in this area, but neither will your competitors be able to get an edge over you. You and they will compete in other areas, while benefiting mutually in this one. [...]
“Won't programmers starve?”
I could answer that nobody is forced to be a programmer. [...]
But that is the wrong answer because it accepts the questioner's implicit assumption: that without ownership of software, programmers cannot possibly be paid a cent. Supposedly it is all or nothing. [...]
Restricting copying is not the only basis for business in software. It is the most common basis(7) because it brings in the most money. If it were prohibited, or rejected by the customer, software business would move to other bases of organization which are now used less often. There are always numerous ways to organize any kind of business.
Footnote 7: I think I was mistaken in saying that proprietary software was the most common basis for making money in software. It seems that actually the most common business model was and is development of custom software. That does not offer the possibility of collecting rents, so the business has to keep doing real work in order to keep getting income. The custom software business would continue to exist, more or less unchanged, in a free software world. Therefore, I no longer expect that most paid programmers would earn less in a free software world.
It's clear from these quotes that even in the early formation of the free software ideals, allowing commercial use presented an economic hurdle, but not an insurmountable one. Conversely, drafting a license that disallowed commercial use entirely would have (1) made this vision of free software as the core of future commerce impossible and (2) would have run afoul of the entire rest of the manifesto that speaks about freedom to use the and distribute software as one wishes.
There is also a practical problem that "commercial use" is often ambiguous: does it include nonprofits? Even when they are using it to solicit donations? Do governments count as commercial? Is the profit-seeking nature of an agency performing distribution sufficient to categorize some use as commercial, despite no money changing hands for this particular software? In the specific case of the Commons Clause, the prohibition on commercial "hosting or consulting/ support services related to the Software" has this kind of ambiguity: the prohibition on "consulting/support" seems dangerously broad. If I sell a book on how to use the software, or charge tuition on a course about using your software, am I in violation of the license? Possibly, if I include the software with my book, or install the software on the computers I use for the course.
This hazy field of categorization, alongside the strong historic non-monopoly commercial success of major FLOSS projects, has led the community to strongly reject non-commercial terms.