This is mere subjective guessing based on my observations:
Standards are currently climbing upwards on Danbooru. The influx of material increases steadily, more and more rather high-quality pics go unnoticed simply because they are neither stunningly good or especially interesting. To learn the standards by lurking is difficult, since very much of the "could-be-approved-could-be-deleted" borderline material is posted by contributors. In the middle of this flood, GIFs are not a top priority of the board, they get easily marginalized...