Fast bowlers generally are stationed on the fence at third man or fine-leg in Tests because they can unleash powerful throws and because it allows them a breather between overs. They are also kept there because they are not thought of as great tacticians or leaders, and in team huddles, they are not looked at as people who are very vocal. Hence, you seldom see fast bowlers become Test captains.
The exceptions have been Bob Willis for England, Courtney Walsh for West Indies, Wasim Akram and Waqar Younis for Pakistan and Pat Cummins, recently for Australia. Kapil Dev, Shaun Pollock, Ian Botham and Imran Khan were also fast bowlers, but they were also all-rounders of considerable pedigree.