Because they didn't make any effort to treat this like a trade mark until after the term itself become generic to the public. If you want a trademark you need to be careful how you use it and how others use it to ensure it doesn't become a generic term. Once something is a generic term it is almost impossible to get it back. (it has been done: Xerox used to be the generic term people used for make a copy - but most of the effort was their competitors who for obvious reasons didn't want to use their competitors company name as a generic term)

If you want a trademark you need to defend it. That means you know the generic term and use that when required. That means when anyone uses your trademark in a generic way your lawyers are immediately sending letters. Check with a lawyer - there are a lot more details you have to get right.