Because around the time that title was given out, Hungary was somewhat faltering in it's ability to repel the Turks and within 100 years the Ottoman army would be advancing onto conquering Buda and Pest. Eastern Hungary was already starting to erode under Ottoman advances and this was prevented on more than a few occasions by Polish forces.

The title is more of a retroactively applied one however, owing to the Polish contribution at repelling the Ottomans at the gates of Vienna and helping check them on the steppes of Ukraine.

Hungary was having a very tough time around that period because decades of political planning and diplomatic maneuvering failed. The Hungarian Crown lost control of Poland and it's own monarch at the unsuccessful assault on the Turks at the battle of Varna. Hungary had acted as the regional hegemon until that point and was now reduced to political strife and decline that was opportunely exploited by the Hapsburgs.

The Hungarians work for repelling the Turks prior was forgotten when their own failure left much of the Balkans open for Turkish expansion and precipitated a long period of Ottoman dominance in the region.