Nobody knows if it's something that we are deliberately doing or whether it's just because of how things have gone in terms of what we most need combined with the contract situation of our players.
Last season we had to focus on our attack as what we had was absolutely atrocious, by far the worst we've had in PL history and probably for a long time before that and would have done nothing but keep us near a relegation battle if we didn't completely rebuild it. While our midfield and defence had their weaknesses it was nowhere near as bad as our attack. Despite that, because of the player's ages we had teams willing to take a chance on some of them and give us a decent amount of money which we were able to put towards the rebuild. So focusing on the attack both bought in the most funds in sales while also making the biggest improvement on the field.
This season due to Casemiro being out of contract and Ugarte having proven he wasn't an option (and then getting the injury), midfield is clearly the huge priority. I certainly would have liked to have get at least one defender in as well (especially LB), but that could only happen if we still had funds left over after the midfield rebuild.
Next season Maguire and Shaw are out of contract and also one year older, Martinez is actually out of contract as well although there is a +1 we will likely take, we'll have a better idea if De Ligt is able to recover from his injury, and also whether Yoro or Heaven look like they are getting near the point of staking a claim for a starting role. So our current squad make-up naturally points to a heavy focus on defence next season.
After that hopefully we'll start spreading things around much more.
I did mention about the sales we were able to make with our attackers last season, but it also goes the other way. Unless you are happy to sell Mainoo, Yoro, Heaven or Bruno, we didn't have much in the way of sellable assets in midfield and defence. I'm sure we could have sold Mazraoui for a bit, but nothing major. Dalot is perhaps the only real option where we probably could have sold for a decent amount to put back into the squad, so an argument could be had that we should have taken that option. But there's also a reason why he's the only one we could have got decent money for, and that's because he's the hardest one to replace. Otherwise, we're basically stuck with the others until their contracts run out or they prove they have recovered properly from injury.