The Who and What relationships on Task are not normal relationships -- they are polymorphic. This means that they can refer to different types of objects.
In the case of Who, it can refer to a Lead or a Contact. As such, you can only get to a limited subset of the fields in the relationship, namely the ones mentioned in this doc page.
To get to other fields on Lead, you'll have to query the Task first to get its WhoId, then query Lead where the Id equals that WhoId value to get those other fields.