The decision to purchase the aircraft (64 in total, with 7 two-seat F-18D models and 57 single-seated F-18C models) was made in 1992, soon after the 1991 collapse of the Soviet Union. A key goal in the Finnish foreign policy of that era was to take no action that might be interpreted by the Soviets as a security threat; a weapons purchase of this magnitude certainly applied. Buying a NATO-compatible, American fighter jet was not possible for Finland before the U.S.S.R.'s collapse.
The primary reason for the lack of ground attack features in the aircraft is the semantic meaning of the word "attack". For example, Finland has Defence Forces, not an army -- even the possibility of Finland ever attacking its neighbors is denied on all levels. This made the policy decision to purchase attack aircraft impossible in the nineties aftermath of finlandization, leading to factory reconfiguration of the F/A-18 to the F-18 variant.
It is widely believed that the removed features can be reinstalled with comparative ease if a regional conflict or a similar need so dictates.