"Doing Time at Leavenworth

There are two kinds of Soldiers at the U.S. Disciplinary Barracks at Fort Leavenworth, Kan. — the ones with no rank or pay, and those with keys.
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The USDB, unofficially called "The Castle," is the only maximum-security prison within the Department of Defense. Of 440 male inmates, six are currently on death row and 10 are serving life without parole. Female felons are locked away at the Naval Consolidated Brig in San Diego, Calif.
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Despite prospects of moving to a lower-custody grade for good behavior, some inmates remain in maximum security for most of their stay.

The staff’s intent is for all inmates to join the general population. Medium- and minimum-security areas contain large open spaces where inmates may spend free time.
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The USDB has operated since 1875. Sexual offenses currently account for more than half of inmates’ crimes. The last execution was conducted April 13, 1961."
http://usmilitary.about.com/od/justicelawlegislation/a/leavenworth.htm

So, first, it doesn't have the capacity. Second it's for soldiers. Third it's a prison- prison is where you send people after they have been determined they have committed a crime. In the case of Leavenworth the prisoner are mostly US soldiers. Gitimo is a jail, for prisoners of war. Prisoners of war are not necessarily criminals. Prisoner of war are generally released after a war is over, they can also be released during a war as part of agreements among the combatant nations.
POWs which are not legal combatants [such as spies, special forces, etc] generally have different standards as would certain types high ranking enemies- such as, if there is possibility of them guilty of war crimes.
Finally, the prisoners at Gitmo could be targets [to break them out or kill them] for terrorist organization. In gitmo there is no local population from which an assault could launched against the facilities.