From [Doc][1]:

> When creating Custom Fields for Activities, You will find that Rich
> Text and Long Text Area Fields are not available.This is expected
> functionality, at this time, you cannot choose these types of fields
> for creation.

**Workaround:**

Possible work around would be to create Additional Text Area Fields allowing for greater amount of data to be stored in Activities at one time.

**Idea:**

Allow custom "Text Area (Long)" fields on Activities.

  [1]: https://help.salesforce.com/articleView?id=000182305&type=1