presumably the goal is to have the smoothest and most consistant combustion, and therefore less vibration, and known ,predictable resonant frequencys in the system.
Which makes very small increments of time important.These rockets are consuming ? tons/sec of fuel, and then its a question of how "small" of an inconsistancy will break things.
A simple but true engineering pricipal, is that any noise, is proof of ineficiency, and therefor a full examination of the audio frequency , coupled with video of the flame.Other data that might be nice would be to know exactly what is happening inside the turbo pumps,and fuel lines.
The whole thing is so violent, that resonant phenominon not seen anywhere else, could be doing very weird things inside tanks and lines. And sensors for that, especialy video, could be challenging to engineer.
Here is a wierd fact about a high pressure diesel fuel line: the pressure pulse (-+ 10000psi) causes
a travelling expansion in the fuel line, that then causes a magnetic pulse, which can be detected, but you want to be quick about it, eh!