It was still weird, since she was known for so long.
It's not weird at all because this has happened to multiple of them. Megu and Kasumi both appeared in game months (longer in Kasumi's case) before being made playable, and got much more attention after being added to the playable roster, and they had a much bigger presence than Ichika did, who was briefly in like one cutscene.
It can't be that shocking that a character that barely appears isn't going to leave much of an impression beyond, "I can't wait for her to be playable."
It was still weird, since she was known for so long.
Basic rule of lewd fanart, most artists don't actually play/read/watch the media they produce art for, as such they only learn about characters when there's some kind of announcement. Like how Asuna fanart exploded once the Bunny version was announced, despite her being in the base game. If a character solely appears in a story update they don't get an announcement, meaning most artists don't learn about them, and as such there's not a lot of fanart. Watch how we'll get tons of fanart of Erika and Kirara once they finally become playable.