Wages that may have been acceptable at one point, may become not acceptable over time as the world around changes. The same with work conditions. One could say that with any job, if you don't like the wages/conditions, go get another job. And if that reasoning applied to every occupation in dispute, there could be a mass exodus from some socially important occupations.
Not everyone is in the position to leave a job to get another for many reasons though. It is an easy thing to say though.
With regards to junior Doctors in particular, to many of them it will have been seen as a calling, and that has been preyed upon to be treated and paid badly for many decades. It takes a lot of time to become a Doctor, so that is many years of poor treatment and not very good money. Be thankful that most of them put themselves through it all, or we would all be stuffed, as there are Doctors available in the UK for most people. Their working hours/conditions will have hopefully been reviewed over the years and been improved, and their pay needs to be too, as the cost of living rises massively, as well as their debt levels for such a long time training.
If workers did not challenge the pay and conditions of the jobs they are in when they think it is warranted, pay and conditions would never improve if left to most employers/business's, or in the case of Doctors, employer/Government.