The Boeing 757 was the highest-capacity and latest-designed narrowbody Boeing ever produced (and, in the case of the 757-300, the highest-capacity narrowbody anyone ever produced); it was also the only Boeing narrowbody lacking any form of manual-reversion capability for its flight controls in the event of a total hydraulic failure.
Now, I can sort of see why Boeing didn’t bother to give their widebodies manual-reversion capability (even if I don’t agree with their reasoning on that point), but the 757 is just a large narrowbody, and not even Boeing's largest; the 707 was even heavier (123,830 kg MTOW for the 757-300, versus 151,500 kg for the 707-320B/C), and it had full manual backups for its flight controls. Also, in the time since they decided to leave manual reversion out of the 757, they’ve grown the 737 to near-757 size, and yet haven’t had any trouble at all keeping its manual-reversion system working!
Besides, if our friends beyond the Iron Curtain could do it all the way back in the 1960s (MTOW 165,000 kg)...
The only reason I can see that would sort of make even a bit of sense (if you scrunch up your eyes and look at it in a certain very-specific sort of way) would be if they left it out in order to avoid introducing excessive differences between the piloting skills required for the 757 and those required for the (widebody, and, thus, reversion-free) 767, and thereby possibly jeopardising the common type rating pilots can obtain for the two - but how can operational convenience possibly justify skimping on safety?
Why did Boeing leave manual-reversion capability out of the 757?