He hasn't made a public appearance for quite a while, which means he could have potentially been dead or kept in a vegetable state until it was too late to elect a new senator this cycle. Since it's too late to elect someone, the Governor of Arizona (Doug Ducey (R)) will appoint a new senator to take his place until the 2020 elections. This means if the Doug Ducey is a swamp creature, he could appoint another swamp creature to office. This also means whoever he appoints will have a much better chance of winning the next elections.

It's also worth noting that the new senator will have to be a republican to match the party of the deceased senator. [Note: Although John McCain was actually a democrat, they still have to match it to the party they're registered as.]