My Seiko Kinetic doesn't hold a charge as long (or at all)as it did
when it was new, why?

When the Seiko Kinetic watches first appeared the charging
capacity was only 2-3 days.  Technology has pushed this
limit to around 21 days (or more).  Consequently, if your
watch is over five or six years old the capacitor (rechargeable
battery) might need to be replaced because of leakage (flaws
in how the cell was sealed).  The newer capacitors are
designed to better resist leakage and increase storage capacity
(even if your watch only had a 2-3 day run time).  Keep in
mind, if your capacitor has leaked your watch will probably
need to be serviced (overhauled).