They're really two quite different things despite any superficial similarities. In the one you have a very limited set of letters to choose from and can form any word you want, in the other you have all the letters of the alphabet, and then some, in infinite supply but you can only put in the ones that belong.

I am good at crossword puzzles and terrible at Scrabble, but I can often do the Jumble® as though I were reading the answers from a list. I can do ScrabbleGrams® fairly easily if they use all the letters provided but have trouble if the word they are looking for doesn't use all the offered letters.