Why should this be? It's not as if the Welsh have the English worry about looking imperialist if they start waving flags too vigorously. There's plenty of national pride. And I'm sure Felinfoel Stout tastes just as good as Guinness.
Maybe the problem lies with St David himself. Hardly anything is known about St David, except that he was Archbishop of Wales in the sixth century. He didn't convert anyone, he didn't get martyred, and even his most famous miracle (that of making a small hill to stand on so that the people who'd come to see him preach could get a better view) is rather - well, unexciting. Whereas St Patrick has so much going for him: the slavery, the snakes, the debates with Oisin, the facing down of druids left right and centre.
I have a proposal. St Patrick (according to my Oxford Dictionary of Saints) was born in western Britain, somewhere between the mouths of the Severn and the Clyde. In other words, there's an excellent chance that he was in fact Welsh. So, get Dewi to move over, make St Patrick's day the Welsh national day, and paint the town (and the shamrocks) red on March 17th!