Because Poland was the Iraq of Europe - the fertile crossroads of different cultures and political influences. Poland as a nation state came and went like the tide depending upon whose Cossacks, Hussars, or Cavalry rode roughshod over its borders.

As a result, there were numerous “:legitimate” claims to Poland, but then you have the true problem: Poland lay as the middle ground between Germany and Russia, so whoever controlled it possessed a strong buffer between its adversary. The Western Allies all knew full well how post-war Europe was going to shape up, and knew of the need to limit Russian incursion into this buffer zone. Unfortunately, there were multiple Polish governments in exile (the two main ones propped up by Russia and the Western allies, respectively) so it was not entirely clear what should be done.

In the end, the Western Allies caved to Russia so as to preserve the alliance, and thus Russia got a solid buffer to its borders with the West.

Although not essentially a reason why Poland was so important, but why it became so important at Yalta was timing. At Teheran most talk was about the allied grand strategy, with talk about the post war world being postponed until Yalta. By the time of Yalta (Feb 1945) it was becoming a pressing concern because the Red Army was advancing into Eastern Europe and towards Berlin. Thus, for the reasons explained in the other answers, Poland became a pressing question.

In addition to what has already been written, Poland was especially important to the British as Britian went to war in September 1939 to protect Poland from primarily the Germans but also the Soviets. It was kind of pointless going to war over Poland if you were only going to hand it over to the Soviets 6 years later.