Lots of reasons that work together -- not one big reason.Why did our midfield get worse?
Guys like Barlow, Crowley and deBoer going out over a period of a couple of years took a lot of defensive grunt and contested ball hardness out of the midfield. Combine that with Fyfe missing most of 16, then (naturally) struggling a little in 17 before missing a fair chunk of 18, and our "big bodied" midfield simply hasn't existed for a lot of the last 3 years.
Sandi playing less than 50% of games over the last 3 games has meant plenty of games when we have been beaten in the ruck.
Lack of defensive pressure in forward 50 has meant our mids have often been on a hiding to nothing when the oppo half-backs get the ball and can run & spread out of defense. This has been because we don't have the running power in our small forwards. For all his faults Ballas is probs still our best 2 way forwardline runner & no one has replaced the tackling and pressure of Mayne.
Finally, we had some v.good midfield coaches and so our stoppage structures and tactics were great. These days we don't seem to have the same tactical nous.