Many theories have been passed as to why including -
a) Louis XIV, king of France, who preferred IIII over IV, ordered his clockmakers to produce clocks with IIII and not IV, and thus it has remained.
b) Many clocks use IIII because that was the tradition established by the earliest surviving clock, the Wells Cathedral clock built between 1386 and 1392.
c) With IIII, the number of symbols on the clock totals twenty I's, four V's, and four X's, so clock makers need only a single mould with a V, five I's, and an X in order to make the correct number of numerals for their clocks.
Take your pick. No-ones really sure