Why did nationalist ideas differ in eastern and western europe?

I assume you are referring to the 19-century nationalisms. Nationalist ideas differed in Eastern and Western Europe primarily because of different political circumstances. The common point of all European nationalisms in the 19th century was national unification. For example, the fragmented, small countries on the Italian peninsula sought to unite into a single country because they realized it was their common interest, as in the previous centuries they were separate and more vulnerable to foreign threats. It was a similar situation with German people, who wanted to unite and strengthen their position.

On the other hand, Eastern Europe was settled by Slavic peoples, many of which had lived under the Ottoman Empire. So, their nationalist cause was intended towards liberation from the Turks in the first place, and only then towards creating unified states. For example, the Balkan nations had lived under the Ottoman rule for centuries, and in the 19th century, they managed to overthrow the Ottomans and achieve independence.