I think it's the size of the ES query object as well as complexity... even the ES documentation provides SQL examples to explain the ES query object...

Here's a good example:

https://www.elastic.co/guide/en/elasticsearch/guide/current/...

SQL:

     SELECT document FROM products WHERE productID = "KDKE-B-9947-#kL5" OR (productID = "JODL-X-1937-#pV7" AND price=30)

ES Query:

    {
   "query" : {
      "filtered" : {
         "filter" : {
            "bool" : {
              "should" : [
                { "term" : {"productID" : "KDKE-B-9947-#kL5"}}, 
                { "bool" : { 
                  "must" : [
                    { "term" : {"productID" : "JODL-X-1937-#pV7"}}, 
                    { "term" : {"price" : 30}} 
                  ]
                }}
              ]
           }
         }
      }
   }
}