Why do Saints Perform Less Miracles Over Time? Anything is welcome, 
but I would prefer something in accordance with Catholic theology. 

I think I already know the answer but perhaps I am wrong, and I'm sure many Christians are wondering. 
I've been noticing that saints do miracles especially when canonization is on stand-by but then afterwards they tone it down. 
I'm guessing it has something to do with the importance of canonization but also their merits as saints.
Rome sometimes publishes the miracle accepted for a canonization, but not always. Who knows how many Rome looked at? Besides that there is no data bank for gathering claims of miracles. Lourdes is an exception and all claims are investigated by a board of doctors.

Here is a list of approved miracles of Lourdes. Almost 7,000 cures have been documented at the waters of Lourdes. The Church has vigorously investigated and validated a mere 67 of them.