Notice that she is indeed called King David's wife. This is an idiomatic way of identifying her. Just as אֲחִינֹ֙עַם֙ הַיִּזְרְעֵאלִ֔ית is identified by where she came from, אֲבִיגַ֥יִל is identified as what she was known as, אֵֽשֶׁת־נָבָ֖ל הַֽכַּרְמְלִֽית. In this case an idiomatic way of saying "who had been married to נבל, and who was known as הַֽכַּרְמְלִֽית"
Note that in Shmuel I, 25:2 we see that Naval's main business was in the Carmel area, even thoug he lived in Maon. When she went to marry David, in 25:40, it says
And David's servants came to Avigayil, to Carmel, and spoke to her saying, "David has sent us to take you to him for a wife."
Had she not been married before, then only the location would have been used.
In this particular case, since she had been married before, the information is included just as we would have in the kesuvah.
Another example would be if the identification was "The red head from Carmel". It is a matter of how she was known by the people around her. In her case she was known and referred to as "Naval's former wife from Carmel".