Why would an organized bus boycott put economic strain on the city?

It would of course cause bus attendance and revenue from bus tickets to drop significantly.
Usually boycotts are initiated in retaliation of something for example the removal of Rosa Parks for taking a white person’s seat. Boycott’s are really effective in sending a certain type of message across, such as wanting equality in bus seating, which later translated into the end of Jim Crow laws altogether. Another example is the Boycott of Goya Food products because the Goya Foods President who is of Latin origin, openly supported President Trump.