Because the question isn't about the strength of the individual bowlers. The question is about which attack is better: Haze/Starc/Cummins/Pattinson or Haze/Starc/Cummins/Lyon (or Pattinson in there for someone if you prefer). Maxwell is a handy bowler but he's no replacement for Lyon, and the ability to bowl a lot of overs, keep the runs down, exploit varying conditions etc is hugely important. Like I said, consider the 2009 Ashes. Australia dropped Hauritz for Stuart Clark for Headingley. Is Clark a better bowler than Hauritz? No question. So were Johnson, Siddle and Hilf at the time. That's a pretty damn good 4 man pace attack. But it still arguably cost Australia the series that they didn't pick a spinner for the 5th test. They ended up bowling Marcus North from one end anyway, and much like in that situation, in any situation where Maxwell is bowling a lot of overs you're going to want to have Lyon in the side instead. Saying "well if there's turn we have Maxwell" is like saying "if it seams we have Henriques" or something, it's ignoring the massive gulf between a quality specialist and a decent fill-in bowler.
An all pace attack works fine when there's no other option, or when there's life for the bowlers and they can knock a side over before the second new ball gets old consistently. If there's turn, the pitch is flat, there's a partnership to be broken etc you want options. As long as you have a quality spinner and the pitch is anything other than a straight up seamer, it's a no brainer to pick one IMO, and Lyon has been incredibly useful year after year. It's crazy to me to suggest Australia would have been better off without him in previous summers. I don't see why this year would be any different.
edit: That said I do think there might be occasions when you'd want to leave out a spinner, but it's a rare event predicated on very specific conditions. The vast majority of home pitches, anywhere in the subcontinent or the West Indies etc you're going to want Lyon.
