For many years, there was a waiting list to get season tickets. The stadium expansion, TV overload, lack of championships for twenty years, pricing and declining household living standards, ticket price increases and mandatory 'donations' (LOL), etc all contributed to a real softening of the demand. I think the sell out was likely for 2018 but another dismal season and it would have meant some ticket price DECREASES, etc to move them I am sure.
Selling the tickets vs. attendance - that is another discussion really. Many long time fans bought tickets as a donation in and of itself as they knew the market value of the ticket was much less than they paid for them. Most fans who actually attend the games are die hard / dedicated followers and willfully 'donate' nothwithstanding the alternative uses for the funds, etc. A generation has now grown up without a championship team. This will have to change soon or the tickets will remain an easy get. In recent years (last 10 or so anyway) you could almost always buy a couple tickets for less than half of face or even as little as $10 each.
Scalping was not always legal in Lincoln, atleast oustide the stadium. I am not so sure they pay much attention to this now as the selling outweighs the ethical concerns some would have on it.