With sanctions the US can declare that third parties must choose between doing business with the US and Cuba / ZTE. Because of it's large economy the US would be confident that most of those parties would choose to choose the US over the company / small country.
Antitrust enforcement is many ways more difficult than sanctions enforcement because third parties must be compelled in 'how' rather than 'whether' they do business with a company.
Even the US would've struggled to break up ZTE / Huawei etc into smaller companies across every jurisdiction for antitrust. Doing so in China would've obviously been impossible but even enforcing this in the Gulf States & South Korea would've stretched the limits of US power.