Italy is just of lesser concern and priority than the Ottomans.
As well it should be. I can't really blame D3 here. Maybe I'm being too kind to the guy because I like him but I can certainly appreciate how low northern Italy is on the list of priorities, especially when there's a war with the Ottomans in less than three years. Those of us with historical foresight gleaned from the third-person omniscient narration of the updates have a better picture of what's going on than the men and women on the ground, especially in an era before rapid communication.
Hostilities with the Ottomans resume February of 1641. It is autumn of 1638 when the banking scandal hits. D3 and his family/staff have basically 2.5 years to A - at least try and fix the economy, or at least get it to the point where people begin to trust banks again; B - mobilize, pay for, and supply at least one theater-sized army in Northern Mesopotamia for at least one and possibly more campaign seasons and C - do all the other stuff they have to do (deal with the Triunes, the Spanish, the power vacuum in Central Europe, all the men on pensions/their widows, and so and so forth) as well The earth doesn't stop spinning after all.
Oh, and to top all that off? There's a very real possibility - hell, even likeness - that D3 dies before the truce expires. So Rhomania will have to do all that stuff and deal with a new Emperor as well.
Is it annoying that Rhomania snatched defeat from the jaws of victory because they took their eye off the ball? Yeah, it is. Is it something that is completely realistic given the absolute cluster that is the situation as described in the last half-dozen or so updates? Yeah, it is.