Why can't Iron Man travel to outer space with his suit on? What is the science behind that? Can't he use the material used to build the space shuttles to build his suit and travel into space?
He actually can, just not with all of his suits (just as not all of them are supposed to be used underwater, in some vulcano, etc.).
As far as the MCU is concerned, Iron Man 3 introduces the Mark 39 (aka Gemini or Starboost), which according to the Iron Man Wiki on Wikia:
The Mark 39 has the unique and special ability to fly at extremely high altitudes, and even reach into space. The armor is advanced enough to go out into the sub-orbit of Earth and the very vacuum of space itself.
And Iron Man 3 - JARVIS: A Second Screen Experience:
Sub-Orbital Suit designed for off-earth missions. Armor features integrated removable Booster Pack, specialized Maneuvering Thrusters, and heavy gauge Heat Shield.
From the Image Gallery on Marvel.com
He can travel into space, as shown in "Marvel's Avengers", via a portal. However there's still going to be limits to how long he can be there, as also demonstrated.
The Iron-Man suit appears to be pressurised enough for him to survive the exposure to the vacuum of space, but the suit doesn't appear to have enough oxygen to support him in space.
This could be argued against from the scene where he's doing underwater welding in the suit, but that might extract oxygen from the water to sustain him.