In 1976 Commodore as a company had been running for more than twenty years and had extensive experience with designining and selling consumer electronics products: they were one of the top-selling electronic calculator manufacturers of the early 1970s. The team put together to design the PET also had at least one member, Chuck Peddle, with extensive experience in computer design and engineering. (Peddle had worked on the 6502 CPU itself, and also designed the KIM-1.)
Commodore seems to have started design of the PET in the summer of 1976,¹ and they had a prototype working well enough to demonstrate to the public at the Winter CES in January 1977.
Yet Commodore didn't ship the PET 2001 to consumers until December 1977,² eleven months after the prototype and about eighteen months after design had started.
By contrast, Tandy had engineers with less experience and budget and didn't start design until December 1976, yet Tandy was shipping the TRS-80 by Septemember 1977.³
So what happened to delay Commodore so much in getting the PET 2001 into volume manufacturing, when Tandy managed to do the same with a similar system in only about half the time?
¹"Jack Tramiel gave Chuck Peddle six months to have the computer
ready for the January 1977 [CES]." —[Wikipedia]
²They did ship a hundred review units in October 1977.
³The initial quanities were low, but that seems to have been due not to inability to manufacture more computers, but due to incorrect sales forecasting resulting in an initial manufacturing run of only 3000 units.