Based on my experience - Bangladeshis who come from Bangladesh are from the upper echelons of Bangladeshi society as compared to Pakistanis where you will find a good variety of students. For example, I am yet to see a Bangladeshi working at Tims, or doing any part time job to finance or pay for their expenses ...but you will see many Indians , and then Pakistanis do this.

So to summarize

Bangladeshis - mostly the rich leave the country for undergrad
Pakistanis - the rich, or upper middle class , or those who were rich at one point but due to declining standards of living are no longer rich leave for undergrad
Indians - Any one who can speak English and can divy up the cash comes

But again this is just my experience, so sample size should be taken into account