Hmmm... how to put this...
Scripture does not interpret itself. If it did, all Protestants would be united in one giant, doctrinally monolithic faith, even more united and doctrinally certain than Catholicism.
But there are many divisions in Protestantism over how to interpret the Bible, some of them minor - like the question of whether immersion is the only valid form of baptism, or of whether the Eucharist is the genuine article or just a symbol - some of them major, like Calvinism as opposed to Lutheranism or Arminianism.
All of these divisions are caused by different interpretations of Scripture. Some Protestants and some Catholics are able to overlook some of these differences, not being certain what is dogmatic and what isn't. But some issues are simply irreconcilable. For example, baptism by pouring can't be both valid and invalid at the same time. It has to be one or the other.
So what do you do when two dogmas clash? What do you do when two people get into fisticuffs? You call out a policeman to settle the score until the trial.
But who is the policeman? Who is supposed to make the call when two groups hold opposing and contradictory dogmas - trivial or otherwise?
Or, as more often is the case, very often two sides both have an incomplete understanding of the Bible. Take
Calvinism and Arminianism.
Who is to say one or the other is correct? Who can say with certainty neither is completely wrong and both have some correctness? That is, who has the authority to sort out contradictions, or apparent contradictions?
Policemen and judges sort out who is guilty or not guilty.
Who are, on Earth, our policemen and judges?