Something else to remember is that this wasn't solicited to comic shops in the usual manner. Omnibuses can be solicited in the Diamond catalogue up to 9 months in advance, and retailers would normally have about 2 months before final order cut off. For example, the Jeff Lemire Animal Man omnibus was released at the same time as this Flash Omnibus, and was in Diamond's March 2019 catalogue.
I'm not sure why, but retailers were only informed about this omnibus in late September, and they only had three weeks to get orders in. It's entirely possible retailers missed this.