anyone got any idea, why its like that?
Case particles (格助詞) indicate the roles of words in a sentence. が indicates the marked word is a subject, を indicates the word is a direct object, の indicates the word is an owner, and so on. However, は is not like case particles in two ways:
は can mark words regardless of their case. In 私は走る, は is marking a subject. In リンゴは食べる, は is marking an object. In この町には公園がある, は is marking a location. In all these sentences, は is specifying the topic of a sentence. A topic is a word around which a sentence is constructed, and it is different from a subject.
You may notice は is syntactically similar to English "even" or "also", in that they can attach to almost any word regardless of whether it's a subject, an object or whatever ("Even/Also I run", "I eat even/also the apple", "I go even/also to Japan", ...).
は basically works by following another case particle (からは, へは, には, までは, etc.). が and を are exceptions here; these two are shy and somehow hide themselves when は comes closer. When the subject (normally marked with が) is also the topic of a sentence, you have to mark it with just は, not がは. The same is true for をは (although をば remains in exceptional cases).
So the purpose of は is not to indicate the case (格) of a word, but to add some "extra meaning" to a word that already has the case indicated by another particle (again, が and を are exceptions here). This is why は is not a case particle but an adverbial particle (副助詞) in Japanese.
も and すら are other examples of 副助詞. They attach to a word of any case and add some extra meaning (namely "also" and "even", respectively). Again, が and を become invisible when these approaches, but if you think a transparent が/を is still there, it may help you understand the difference between 格助詞 and 副助詞.