Sad to see Lambo's absence if it's true but it does make sense from a business POV. High-end cars manufacturers at large-scale shows can't just display their models only; they need to set up private lounges for their existing customers. Some have VIP status so everything that goes on inside the lounge has to be topnotch and obviously costs add up quickly. It's a huge undertaking. For a relatively small company such as Lamborghini, their resources would be much better spent on doing events with a targeted audience of existing and potential clienteles. Chances are Lambo would much rather entertain customers to come for a factory visit than to go through the logistic nightmare and high costs of doing a generic show in a another country. True that with the absence, some potential customers may look to other brands; but targeted event aimed for devotees to the brand is likely plenty more effective and have much better ROI. Dedicated brand events have much better control (and success) over generic trade shows for the public.
I think Japan is a good example and set a precedent for Lambo. They had pulled out of the biennial Tokyo Motor Show years ago. TMS is considered a major automotive event in Asia. But Lambo has been busy doing events of their own within Japan with huge success (vintage model tour in Kyoto, 50th anniversary of Lamborghini's in Japan etc...) and obvious Lamborghini sales in Japan has not been compromised (Japan is still Lambo's 2nd largest market, after the US). So it makes business sense for Lambo to concentrate on Quail. There may also be some business politics involved; now that Porsche has taken over Audi to be Lambo's parent. Porsche remains to be one of the most if not the most profitable automotive brand so in order to prevent Lambo from infringing onto its market share (Lambo is following the same path as Porsche with the introduction of the Urus and the rumoured 4th model being a 2+2), perhaps Porsche is mandating Lamborghini to dial it back a bit?