Consider the following hypothetical decisions and outcomes:
1. std.process is left at is. One user is angry / turned away because it performs 0.1% slower than it can be.
2. std.process is rewritten to minimize allocations. Code complexity goes up, new improvements are challenging to add; bugs pop up and go unfixed for a while because fewer programmers are qualified or willing to commit the effort of making correct fixes. More people are angry / turned away from D because its standard library is buggy.
Of course, the above is an exaggerated illustration. But would optimizing all code left and right really make more D users happier?
There's also the question of priorities. Would you rather than effort is spent on optimizing std.process (and dealing with all the fallout from any such optimizations), or working on something that is acutely missing and hurting D?
This is surely an exaggeration.D is a systems programming language, there is hope that it will penetrate a wide range of systems and environments - sure in many cases a little bit of memory use or performance loss is unimportant, but for many it will be the decisive factor which makes D unusable there.
D does not attempt to please everyone out there who is choosing a programming language for their next project. There is no such language, nor can one exist. One has to accept that D has a number of goals, none of which are absolute, but merely point towards a certain, but not overly specific, point in the multidimensional matrix of trade-offs. D never was about achieving maximum performance in all possible cases.