There’s a lot of demand, and her merch sells really well.
A lot of times, it takes a while for the publishers and editors to realize how popular a character is becoming. It’s never that they hate a character and don’t want her to be popular, it just takes a while for the message to filter upwards.
I think an Ivy book is inevitable, I would be surprised if there’s not one in the planning stages already (I have no insider knowledge on this whatsoever).