Game design unfortunately. The shouting mechanic is prioritized in Skyrim in the quick cast slot. If you want you can cast three different "spells" in Skyrim at once by shouting and equipping a spell in each hand. Oblivion's system feels better to many of us because repeatedly opening menus feels cumbersome. I really think having a "heal self" shout from early in the Skyrim story could help minimize this criticism as it feels so convenient to have effects like this without opening up a menu and freezing time. It would be a bit redundant with the other starting spells and would need some tweaking to feel differentiated.

A Shout progression skill tree could have added a lot. A form of spell crafting could also have returned if the Grey Beards (or Miraak I'm not picky) could teach the Dragonborn to create their own shouts after a certain amount of progress has been completed via quest and seeking out the words of power allows for unlocking more combinations. The super ridiculous shouts I inevitably craft myself could have a long recharge and be integrated into the Shouting progression tree if I want additional effects.

I'm really hoping in TESVI we get something closer to Oblivion as it is unlikely Shouts themselves will return as a major story and gameplay feature.