I am assuming that when the "creators" are mentioned regarding Margaret Atwood's "The City Planners" you are referring the the city planners themselves. Assuming this, the city planners remain to be the only ones named in the poem because they are the ones the speaker specifically has an issue with.
The speaker blames the city planners for the "pedantic rows, [and] the planted sanitary trees." The speaker has a problem with the "same slant" of the roofs, the "too-fixed stare" of windows, and the "straight swath" of cut lawns. For the speaker, no one else is responsible for the sanitary look of the city. Therefore, there is no reason to bring up anyone else but the planners.