Partly because science and advances in knowledge explain what we formerly considered miraculous.
When the Albanian Theotokos here began to weep and exude myrrh, the press asked "have you tested the tears?"
The priest responded "they did that to the icon in New York a few years back, and they found out that the liquid had the same chemical composition as human tears. What did that prove? Either you believe, or you don't."
Only an evil generation seeks a sign, and this generation is quite cynical and hard to impress. There have been several miracle cures, for instance, which the most modern medicine have proved are without explanation.
And btw, the Church has always been skeptical of miracles. The Fathers warn, if you see one, ignore it. When our Arab Theotokos began to weep here, the first thing the bishop did was not to venerate it, but to exorcise it, so as to make sure that another "miraculous" power wasn't at work.