At first glance, I would have said that the reason it is was is because there is always singular.
However, after looking at the linked question, I have to say that it agrees with the first item.
- There was an apple and an orange on the table.
correct
- There were an apple and an orange on the table.
questionable, non-standard?
- There was some apples and some oranges on the table.
questionable, non-standard?
- There were some apples and some oranges on the table.
correct
- There was an apple and some oranges on the table.
correct
- There were an apple and some oranges on the table.
questionable, non-standard?
- There was some apples and an orange on the table.
incorrect, maybe non-standard?
- There were some apples and an orange on the table.
correct
The reason that those cases of "there was" seems questionable and not entirely ungrammatical is because there is singular.