In many PhD projects, you have to write your own research proposal. This is in some ways similar to the process that professional academics go through when applying for funding to do research.
It is not enough to have a good idea and research plan. You need to be be able to convince other academics that the project is interesting or valuable or useful.
A PhD research proposal is a pitch for investment of resources… whether that investment is money, equipment or time. Before anyone will invest in your project, you need to do two things…
1. get their interest
Of course, different people find different things interesting, valuable or useful. Your research may be very far removed from any obvious practical application, but it can still be of academic interest to others working in the field.
So you need to know who you are selling it to, and why they should be interested. It is this understanding, as much as your technical knowledge, that will help you sell your research idea.
This means becoming familiar with the literature and knowing how your work fits into the broader context, but it also means getting to know people in the field, what motivates them and what they find interesting.
2. reassure them
Any investment carries some risk. If someone invests money in your project, or agrees to invest time supervising it then they carry some of the risk if your project is a disaster.
So you need to reassure them that the research idea is viable, that you have done adequate background research, and that you have thought through clearly how you will carry out the project.
If you can convince other academics that your project is interesting, then reassure them that you can deliver, then you will have a high chance of success.