In farming, monoculture is the practice of raising a single crop over and over again. You can figure out exactly what fertilizers it needs, how much water, and predict your yield accurately, year after year.
Then, one year, a blight or a virus or a bacterium or a weevil comes along and eats everything. If you had planted four kinds of crops, you would have lost 25% of this year -- bad, but potentially survivable. 100% is rarely survivable. In order to make sure the blight is gone, you may need to burn your fields to the ground. That will stop them, unless of course one of your neighbors didn't do a perfect job of burning it out.
Let's switch topics. Herd immunity is the effect that protects individuals who haven't been vaccinated against a particular disease, by a very simple process: anyone who is immune can't pass the disease on. So if a nice large portion of the herd is immune, many of the folks who are vulnerable will not get the disease because nobody gives it to them in the first place.
Most companies use a single operating system for all their desktop computing needs. Mostly they use Microsoft Windows. They only have to support one operating system, so they can get efficiencies in help desks and services. It's so pervasive that you usually don't even have to teach people how to use it, although you often need to teach them specific applications to get their job done.
Then, one year, ransomware comes along.
The companies that built up their immune systems are hoping for herd immunity to limit the spread of the disease. It turns out that many of them don't have quite the robust immune systems that they thought they had.
The companies that are running other operating systems -- Mac OS, Linux, other kinds of UNIX, whatever -- are not participating in the giant monoculture farms. They are completely immune to the problems that the monoculture faces.
Instead, they had to pay for people who could set up and administer the strange systems that aren't Windows. Those people command higher salaries than Windows admins and helpdesk folks. On the other hand, they also tend to handle more systems at a time than the Windows specialists do, because they have been working with automation tools that have only been recently introduced on the Windows side.
It's really hard to change things that people are used to. That doesn't mean it isn't worthwhile, just that you have to account for all sorts of profits and costs to make a good decision.