Why can't we correctly say that all arthropod vectors are reservoirs?

A reservoir for a pathogenic organism (one that causes disease) is typically defined as an organism that serves as a long term host of a pathogen. Most often, the reservoir participates in an essential portion of the disease cycle of the pathogen and/or the pathogen requires the reservoir to survive and reproduce. Reservoirs do not get the disease associated with the pathogen or the infection is subclinical and signs and symptoms associated with the disease are not detected.

A vector is essentially only a transmission vehicle for the pathogen. A vector, such as a mosquito, obtains the pathogen from one organism--an infected host or a reservoir--and transmits it to a new host. The pathogen typically does not spend a substantial amount of its life in a vector (in contrast to its relationship with its reservoir). A vector would only be considered also as a reservoir if the pathogen required the vector for survival and multiplied within it before being transmitted to a new host.

Another way to look at the difference would be to consider if the organism (arthropod in this case) could serve as the source of an outbreak. A reservoir acts as breeding grounds for the pathogen (even if the reservoir does not get the disease itself) and could be considered a possible source of an outbreak. Because pathogens spend only a short time within vectors as they are transmitted from host to host, a vector would not be considered as a possible source for an outbreak of a disease associated with a particular pathogen.