Why is the pronoun “this” ambiguous?

From LitWiki

Only use the word “this” as an adjective, not as a pronoun. For example, in the following sentence, “this” does not rename a noun like a pronoun is supposed to do, but tries to stand by itself as if the meaning is clear:

This causes many misunderstandings.

Of course, the context of “this,” you might say, would be understood from the previous sentence. Sure. However, as a developing writer, do not make that assumption. Only use “this” as an adjective preceeding a noun; e.g.:

This attitude causes many misunderstandings.

Context is still needed for this example to be fully understood, but the subject is now clear to your reader, and she does not have to pause to figure out what in the world “this” is. Review a professional’s writing for “this” — chances are you will see very little of this stylistic faux pas.

Composition FAQ