Why homeowner policy would cover vandalism and not theft?

There is not information in your question to give a comprehensive answer.

But, it just depends on what scope of coverage you purchased and on the nature of your theft claim. For example, Theft by a resident of the home would not be covered. Thefts are required to be reported to the Police to be eligible for coverage.

When you purchased your insurance policy you selected those coverage you wanted. If you just chose the cheapest policy you could find without looking at the details, then you may need to speak with your Insurance Agent about upgrading your coverage.