Because the 787-8's and 787-3's seating capacity is just below the seating capacity of the 767-300 (and the 787-9's capacity is greater than the largest 767, the 767-400). And the 787's seating capacity has crept up over the last year as Boeing has refined the design.
Also, the 787 is heavily targeting the long-range market, and the 757 has seen only limited use in the long-range market.
The 787-3, the medium range version of the 787, is 48% bigger than the 757-200, the primary version of the 757.
The 175-225 seat market will be the next big gap Boeing and Airbus need to address.
Which is why I think Boeing will launch the 737 follow-on sooner rather than later. Let Airbus spend a bunch of money on the A380 and the A350. If Boeing can do for the 150-225 seat market what the 787 promises to do for the 250 to 300 seat market, they will clean up.
I almost have to wonder if these Boeing decisions are part of a grand strategic plan to screw Airbus. Boeing has led Airbus to commit large sums of money after limited markets. Boeing has held its own with the derivative 737 in the largest jetliner market, and Boeing has cleaned up where it has developed new airplanes (777 and 787), forcing Airbus to commit significant R&D to the A350, after Boeing already dominated this market with 787 orders. Now Boeing is reducing the market for the A380 by launching a 747 derivative. It's like Boeing's plan is to force Airbus to commit so much money they will not be able to respond to a new 150-175 seat airplane.
Boeing try to screw Airbus? You think!
The 787 was originally planned as a middle of the market replacement. Whether that is 767/757 or 767 is your call.