Short science fiction in the United States is still bought at a rate per word rather than a flat fee. The convention is inherited, and it continues to shape what writers submit.
The rate came from the pulp era
Magazines that filled a fixed page count each month needed a way to price copy that scaled with the space it occupied, and words map directly onto pages.
Paying by the word let an editor budget an issue the way a printer budgeted paper. It also let writers calculate income before they wrote anything.
That arrangement outlived the pulps and carried into the digest magazines, then into the online venues that replaced much of the print market.
It sets a floor that defines professional status
Genre organizations use a minimum word rate to distinguish professional markets from semi-professional ones, and that threshold governs eligibility for membership and some awards.
A magazine that raises its rate above the line changes category, attracting submissions from writers who were previously sending elsewhere.
Because the definition is a number rather than a reputation, a new venue can enter the top tier immediately by paying at that level from its first issue.
Length categories follow the payment logic
Short story, novelette and novella are defined by word count, and those boundaries exist because payment and page budgeting were always counted the same way.
Awards and anthologies then adopted the same divisions, which means a story's category is decided by its length rather than by anything about its form.
Writers near a boundary feel the pull. A story sitting slightly over a threshold is harder to place, and editors are candid about the constraint.
Editors buy space, not just stories
A magazine assembling an issue has a fixed budget and a fixed hole to fill, so a long piece has to displace several short ones and must justify that displacement.
This makes novellas the hardest length to sell to a magazine, even when the writing is strong, because the opportunity cost is measured in whole other stories.
Dedicated novella lines emerged partly in response, giving that length a home where it competes against other novellas rather than against short fiction.
The incentive on the writer is mixed
Payment per word appears to reward length, but the acceptance rate falls as length rises, so the expected return favors tight writing rather than expansive writing.
Experienced contributors describe cutting toward the length an editor can most easily fit, which is a commercial judgment as much as a craft one.
The result is a body of short science fiction whose characteristic compression owes something to an accounting method devised for filling magazine pages.