Why does history ignore Polands annexation of Slovakia terrority in 1938 when mentioning the attack on poland in 1939? It seems poland had a history of attacking others prior to 1939 events including to Ukriane.

I wouldn't say it is ignored, so much as not really relevant to the overall narrative of the outbreak of the Second World War.

While Poland did annex a part of Czechoslovakia in 1938, it was a relatively small region, Zaolzie, that had been disputed between the two states since they were formed in 1918. It had, or still has, a large ethnic Polish population, and in 1919 they fought a week-long war over it. An international conference in 1920 awarded most of the land to Czechoslovakia, which incense Poland and the Poles in the region.

Thus when they had their chance in 1928 to take it, the Poles did so. This was condoned by Germany for propaganda purposes, as it meant they were not the sole country annexing Czechoslovak lands, and gave the illusion that Poland was working with them. It also worked for the Czechoslovak government because the Germans demanded that land anyways, so it was a means to spite Hitler.

That all said, I'll address your second statement, regarding Poland's "history of attacking others prior to 1939." Yes, they did fight several small wars in the immediate aftermath of the First World War, the most important being the Soviet-Polish War of 1919-21. Poland was not alone in conflicts at this time, it is just that states like Ukraine and the Caucasus republics were ultimately conquered by the Bolsheviks and disregarded as independent states, while Poland was not. But it is also because Jozef Pilsudski, the leader of the Polish state and military, had a plan of a Polish-dominated federation of East European states, much like the old Polish-Lithuanian Commonwealth, to defend against the Bolsheviks. So he was deliberately aiming to conquer large swaths of Ukrainian, Belarusian, and Lithuanian territory, as it was to serve as the bulwark against the threat of a Russian invasion. And in many cases he was successful, as the Second Polish Republic's borders were several hundred kilometres east of the proposed Curzon Line that was to serve as the eastern border (though it ultimately became the basis of modern Poland's eastern border).

Some reading on the subject:

Jerzy Borzęcki, The Soviet-Polish Peace of 1921 and the Creation of Interwar Europe (2008)

Norman Davies, God’s Playground: A History of Poland, Volume II: 1795 to the Present (1982, 2005)

Kevin Hannan, Borders of Language and Identity in Teschen Silesia (1996)

Marcel Jesenský, The Slovak–Polish Border, 1918-1947 (2014)

Richard M. Watt, Bitter Glory. Poland and its fate 1918–1939 (1982)