From Doc:

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.

From Doc:

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.