Here's why:
* Abstract nouns represent concepts, ideas, qualities, or states of being. Speech is a concept, a way of communicating using language.
* You can't touch or physically hold an abstract noun. You can't hold "speech" in your hand. You can hear someone speaking, but that's the act of speaking, not the abstract concept of speech itself.
Some other examples of abstract nouns are: love, happiness, freedom, justice, and time.