La razón de ello

Hola Ελισάβετ

In this sentence "esto" would also be fine. When we are talking about esto/eso/aquello bearing in mind distance, the nuances between them are clearer, but if we are talking about using these as pronouns referring to ideas it then becomes a question of style and formality.

We have a lesson talking about "ello" versus "eso" (although "esto" could also apply here as a nearly synomym of eso). Here is the lesson -have a look. We will though admit both eso and esto in the writing challenge exercise because there is no hint saying that you should use one or another.

Saludos

Inma