I am no expert in Austrian politics, but the success of FPO is consistent and it is periodically mentioned whenever there are elections in Austria.
My understanding is that there are mainly three reasons:
- The Freedom Party of Austria is the main opposition party basically since it exists. That is because the center-left (SPO) and the center-right (OVP) routinely governed together since the end of World War 2 (see Government of Austria). This was probably a necessity given that Austria was committed to neutrality and even today they are not in NATO.
- The FPO grouped everybody, but the Socialists and the Christian Democrats. This included liberals and reformed Nazis. Originally they were more liberals, but in the last decades they moved right. Bottom line, they are used to have a fluid ideology and so are they voters.
- They are popular and they have been for a long time. Which means that many Austrians do not really feel them as a real threat. They may be menacing, but they are not an extra-parlamentarian and violent force.
The last point seems to be true also for foreign governments. They were certainly strong reactions when the FPO first became part of a government after its move to the far right, but relationship were normalized:
In February 2000, Austria's foreign relations cooled down when the ÖVP formed a coalition with the FPÖ after the 1999 election. European governments imposed diplomatic sanctions, and the United States called home its ambassador. The diplomatic sanctions were lifted in September 2000, after a three-member panel assessed human rights and political life in Austria. In November 2000, the United States and Austria normalized their relations.
Foreign relations of Austria
In short, while many people think that it is not acceptable to vote for AfD or FN, it is normal in Austria to vote for FPO. Austrians have elected it in governments and seen that it does not threaten democracy while Germans or French may still fear that AfD or FN might do that.