The Nationals are supposed to represent rural and regional interests. National MPs are blokes in RM Williams trousers and big hats.
They lost their way 10, 15 years ago. A vote for the Nationals has always meant a vote for the coalition, but now it's just a vote for the Liberals. I'm not surprised their share of the vote has fallen away. In the 2016 election in WA they got only 3% of the vote, and the Australian Christians who are a genuine fringe party got 2.6. The Greens outpolled them 4:1. Didn't come close to winning any of the regional seats.
Not sure WA is a great example to use, given a) the WA Nationals are explicitly not linked to the Liberals, and b) Western Australia has a very low proportion of rural population compared to Queensland, NSW (and Victoria). Overall percentage of vote is unlikely to reflect their strength.