Twenty years ago, Halloween was pretty low-key. Falling on the tail end of October between 秋祭り (aki matsuri, fall festival) season and Christmas, it was apt to be overlooked or underplayed.
Halloween also seemed ちょっとコワイ (chotto kowai, a little too foreign and hard to approach). Kowai usually means "scary" or "frightening," but written out like this in katakana, it could point to feelings of nervousness or inadequacy.
Besides, for many Japanese, the "Trick or treat" slogan was hard to pronounce and understand. お菓子をくれないといたずらするぞ (O-kashi o kurenai to itazura suru zo, Give me sweets or I'll play a prank) was the official translation for "Trick or treat" that was in circulation.