Here is the difference, when you ask why you're not testing you're asking for feedback on what you need to work on, how you need to improve, so that you can test. When you just ask if you can test you're not asking for any such feedback and it might even come across as a challenge to your instructor's judgement. The instructor has better judgement than you on whether or not you're ready to test and that's what makes him the instructor and you the student, so if he's not letting you test he has his reasons and as a student I would trust his better judgement, but I would like to know why Im not testing, so I know what I need to work on to meet his standards and hopefully test the next time.
But that depends on the expectations of the instructor. At my dojang, adults are expected to know what's on the test, and to know when they're ready to test. It is your own responsibility to ask. In this case, you aren't challenging the Master, because he has passed the responsibility onto the student. If you haven't tested, it's not because the Master hasn't invited you, it's because you haven't asked.
On the other hand, if you ask if you can test, and he says "I'll check you next class", then if he says you aren't ready, you aren't ready. At that point, it's okay to ask.
Even with the kids, where tests are mostly by invitation, it's okay for a kid to
ask him. It's not that they're arguing that he's graded them incorrectly. It's that we've got probably 150+ kids in our dojang, and it's difficult to properly assess them all during the pre-testing weeks. Sometimes a kid misses a class when we would have seen him, sometimes we just skip over a kid because we think he just got his belt, so he's probably not ready, and he surprises us. Sometimes a kid isn't ready, but really wants his belt, and the conversation turns to what he needs to be able to do in order to test, and they make the extra effort to.
There are a lot of situations, whether your tests are by request or by invite, that it is not an insult to the Master. Sometimes, they're waiting for the "I think I'm ready" before they begin a serious evaluation.
It all depends on the culture of your school, and isn't something universal you're going to find on a generic martial art forum.