Hey there,
Your question as to why BNS has lagged has no definitive answer. That being said, it has trailed for a number of years - in fact, it is the worst performing big five back over the past three, five and ten years.
In my opinion, the reason for its underperfomance is due to the fact that it had an agressive international expansion strategy. Unfortunately, it grew too much too fast and its expansion into the Caribbean didn't work out so well. Likewise, its exposure to Latin America bring about additional risk which is unlike the other four which have focused expansion efforts south of the border.
The good news? In the fall of last year, the company's announced it was refocusing operations. It is now in the process of divesting non-core assets and streamline its international operations with a focus on Latin America. IMO this is a good thing. Over the past few years, it was the Big Five bank with the most acquisitions. As mentioned, I believe it grew too much to fast. Growth through acquisition can be a strong strategy, however it can also be one that sets the company back if synergies aren't realized. Over the past five-years, BNS has the lowest earnings growth rate among the big five despite spending the most on expansion.
The reason why TD and RY tend to outperform, they are simply best in class. They are the biggest, have the most stable and in times of uncertainty tend to outperform. Once again, there is no exact answer here - but this is why i believe BNS has underperformed. I do however, think the renewed strategy is a good things for the company.
Mat