The phrase "developed mainly" is correct and usable in written English.
You can use it when referring to something that has primarily been created through a process of development. For example, "This technology was developed mainly by a team of engineers."
The first two programs concentrate on Latin jazz and salsa, genres that developed mainly in New York.
Ms. Hadid and Mr. Schumacher reviewed and changed numerous options developed mainly by Mr. Pauwels, based on the initial concept.
Health insurance had developed mainly on an occupational basis and was a requirement for that occupation.
The terrier breeds were developed, mainly in England, to rid granaries and barns of rodents.
Dry sausages developed mainly in warm areas where preservation was difficult; fresh and cooked sausages developed in cooler climates.
Although forests cover more than one-third of Venezuela's land, forestry is poorly developed, mainly because the richest forestlands are so remote.
Although dating from the 10th century, the city has developed mainly since the 19th century and has been largely reconstructed since World War II.
The slang of the Gulag eventually became the slang of the entire Soviet Union; the rich vocabulary of Russian obscenity developed mainly in the camps.
Most of the business activities and services are located in the old core and its northward expansion, developed mainly between the 1860s and the 1940s.
Tourism has developed mainly because of improved road conditions and the restoration of the historic Barkerville gold-mining area to the east.
Rājasthānī painting, the style of miniature painting that developed mainly in the independent Hindu states of Rājasthān in western India in the 16th 19th century.
