I believe that is because it is easier to show damages against a company where they can put a numeric value on it (and it is one company) rather than trying to wrangle a class action where the damages are more nebulous (if at all).

As noted in the complaint, there were several parts to it relevant to a browser ( https://www.justice.gov/atr/complaint-us-v-microsoft-corp ). The first three:

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16. First, Microsoft invested hundreds of millions of dollars to develop, test, and promote Internet Explorer, a product which it distributes without separate charge. As Paul Maritz, Microsoft's Group Vice President in charge of the Platforms Group, was quoted in the New York Times as telling industry executives: "We are going to cut off their air supply. Everything they're selling, we're going to give away for free." As reported in the Financial Times, Microsoft CEO Bill Gates likewise warned Netscape (and other potential Microsoft challengers) in June 1996: "Our business model works even if all Internet software is free. . . . We are still selling operating systems. What does Netscape's business model look like? Not very good."

17. But Mr. Gates did not stop at free distribution. Rather, Microsoft purposefully set out to do whatever it took to make sure significant market participants distributed and used Internet Explorer instead of Netscape's browser -- including paying some customers to take IE and using its unique control over Windows to induce others to do so. For example, in seeking the support of Intuit, a significant application software developer, Mr. Gates was blunt, as he reported in a July 1996 internal e-mail:

I was quite frank with him [Scott Cook, CEO of Intuit] that if he had a favor we could do for him that would cost us something like $1M to do that in return for switching browsers in the next few months I would be open to doing that. (MS6 6007642).

18. Second, Microsoft unlawfully required PC manufacturers, as a condition of obtaining licenses for the Windows 95 operating system, to agree to license, preinstall, and distribute Internet Explorer on every Windows PC such manufacturers shipped. By virtue of the monopoly position Windows enjoys, it was a commercial necessity for OEMs to preinstall Windows 95 -- and, as a result of Microsoft's illegal tie-in, Internet Explorer -- on virtually all of the PCs they sold. Microsoft thereby unlawfully tied its Internet Explorer software to the Windows 95 version of its monopoly operating system and unlawfully leveraged its operating system monopoly to require PC manufacturers to license and distribute Internet Explorer on every PC those OEMs shipped with Windows.

19. Third, Microsoft intends now unlawfully to tie its Internet browser software to its new Windows 98 operating system, the successor to Windows 95. Microsoft has made clear that, unless restrained, it will continue to misuse its operating system monopoly to artificially exclude browser competition and deprive customers of a free choice between browsers.

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You can go through these complaints (and the other three - down to paragraph 32) and ask "is Apple doing these things?"

Is apple going out and trying to squash Chrome on windows machines (remember, Microsoft released IE for OS 6 or was it 7?). Is Apple trying to call up Amazon and have them replace Chrome with Safari on their Kindle devices?

The things that Microsoft got in trouble with for its anti-competitive practices with IE are completely separate and distinct from the things that people accuse Apple with for being anti-consumer.

Apple has gotten their wrist slapped for being anti-competitive with books in the past.