As his expenses were not paid for I can see why Nasser would not attend.
But I think it is unreasonable to expect this. That money is better spent on paying competitor expenses, anyway.
Unlike the other ASC past winners on stage, Nasser did not get a formal invitation, or an invitation. He was not mentioned in the video montage and stage presentation. ASC disregarded Nasser as past ASC winner.
It's not about the cost of airfare and accommodation; it's about the integrity of the athlete to the safety of the industry and his/her peers. Nasser is basically giving a bad name to the industry, now that he's retired?

Events like the ASC and the Olympia is affected by the accusations whether they are truthful or not. Many of the product sponsors, who support these events are affected as well.
Like I said "inviting Nasser will be like inviting a disgruntled ex-postoffice employee to the company x-mas party". It's a conflict of interrest and will jeopardize everyone's safety.