I'm at Lansdowne & Davenport and grew up a bit further east along Dav, so St Clair is basically my backyard. I think Corso Italia is often held back by two main factors. The immediate local population of the traditional Italian and Portuguese majority communities have been gradually aging, with the younger generations of those families moving outwards. The elderly who remain may frequent the local bank branches, along with Tre Mari bakery or the Portuguese bakeries near Caledonia, small grocers, or the cafe/sports bar hang out spots, but they typically wouldn't patronize newer type places.
Also for other locals who may be younger, St Clair restaurants often get overlooked because Corso Italia is not really seen as a destination. Bloor definitely gets more action, along with downtown in general. There have been some higher profile places that opened up recently like Diner Seoul at St Clair & Lansdowne, and Savor Thai and Savera Indian Cuisine closer to Dufferin. A greater influx of new residents nearby should hopefully help in the long term.