It's actually more a cultural than a grammatical issue. When a person says "谁说的", it mostly implies the suspicion or the disagreement of the contents or the speaker. It's more an expression of emotions than a real question. Though "是谁说的/了" actually have the same meaning, with one more word " 是" of the falling tone, the emotion is greatly weakened. Try to feel the difference by speaking out those two sentences fast and loud.

“谁说的”can be explained in two ways. 1. It is equal to "是谁说的”. It is used to answer "who" 2.It means you are not agree with others opinion. For example, your friend said " Jack昨天没回家“, if you know what he said is not true, you can also say “谁说的” to explain that Jack went back home yesterday.

“谁说的” is a question and is seeking for an answer. "是。。。的” can be a construction, for example "是谁吃的"，“是谁送的”. "是谁说了?" also ask for an answer but with a little difference because "了“ implies done in the past. From my point of view just a little difference. However the two sentences have it own appropriate application.