Many factors compound into not getting a job as a junior developer. One instance can be, with being a junior developer, there is a lot more that you will need help getting up to speed on and the company may not have the resources necessary to upskill you in a short amount of time in order for you to be productive. Another, can be that you do not currently have enough practical knowledge of a certain tech stack for their need - surface level information is great, however if they need results in a few weeks, they may have better luck getting someone with a bit more experience and understanding of diverse tech stacks and use cases.