Is it mandated to provide root-cause analysis? No, in fact it depends on case to case basis. Every single testing session is different from another and there are priorities you have to deal with as project resources are never limitless.
There may be small apps that don’t require that entire efforts being laid and there are large projects like an enterprise website for example that is quite vital to the organization and HTML/CSS or some other root-cause analysis may be required.
However root-cause analysis is, indeed, a great thing so if your resources allow you to perform it you should at least give it a shot and here is why: it grants you with more information and data is pure power in a tester’s arms. After root-cause analysis testers do understand the app under test in a significantly better manner. You can locate so many new marvelous things like critical areas you have never tested and you will gain additional scope during the analysis. And, surely, there will be more bugs for you to track.
Why is it not followed everywhere?
This process is time consuming and not all projects are capable or willing to spend more additional resources on such tasks, especially if they are low-priority and the projects are small apps or something that is not rich on multiple handy features. There are even times where some team members already know what hides in the root-cause making this activity useless.
How it can be reinstated whether the root cause analysis is needed?
There are several factors, by measuring which you will be certain on whether the root-cause analysis session is something your project can afford. First of all take a deep look at the defect. Is it worth is? Secondly go through your current test coverage as well as your test plan to make sure if you need this additional work in the first place. Then review defect priority to make sure if it requires additional attention and take a good look at your team: are they up for more work? Do they have time? Is it really necessary? And if all the mentioned above factors are satisfying you should go for root-cause analysis.