Why now? Why Shakespeare's "The Merchant of Venice" in 2016? Because money still trumps love. Because racism and anti-Semitism flourish. And because the quality of mercy is indeed strained " which means "constrained," i.e. limited. Portia is talking about how things ought to be, while we (and she) know that mercy is constrained everywhere we look.
SOURCE: Pittsburgh Post-Gazette at 12:00AM on November 8, 2016