Why is Microsoft a global leader in the software industry?

Well, that is tricky to answer.

MS managed to get its operation system (OS) Windows on majority of computers. MS developed a new concept of SW sales. To IBM did not sell his OS, he leased it instead. That was in times, when people thought money were in hardware, rather then in software.

Moreover, despite not first, MS managed to develop and more importantly sell mouse style control of the computer, as well as Windows drag and drop philosophy of working with SW. In some extend we can say that at its time MS was an Apple of that age. He spotted a niche market and developed a new concept of usability that people liked.

And then, it was just a snowball efect. :-)