Because one Iron Man is never enough. 

Tony Stark has been inventing, designing, modifying, upgrading, reinventing, redesigning, remodifying, and re-upgrading the Iron Man personal weapons system over a relatively robust number of years.
 
Through a never-ending cycle of upgrades and specific environmental needs, Stark has created a sizable 'armory' containing his previous models and his continuously upgraded contemporary Iron Man armor.

Made from 100% cotton, our navy-blue Iron Man Legacy Men's T-Shirt features seven particular models -- or 'Marks' --  highlighting Tony Stark's finest -- and wearable -- technological achievements.

 What?? Do I happen to know the name of each model?? 

Of course I know the name of each model! From left to right, top to bottom:

• The War Machine Armor (also known as the Variable Threat Response Battle Suit Mark I). First appearance: Iron Man #281 (1992).
  
  
• Iron Man Mark V: The classic red and gold armor. First appearance: Iron Man #85 (1976).
  
  
• Iron Man Mark VII: The Silver Centurion Armor. First appearance: Iron Man #200 (1985).
  
  
• Hulkbuster Armor. First appearance: Iron Man #304 (1994).
  
  
• Iron Man Mark III: The original red and gold armor. First appearance: Tales of Suspense #48 (1963).
  
  
• Iron Man Mark II: The Golden Avenger. First appearance: Tales of Suspense #40 (1963).
  
  
• The Iron Patriot Armor. First appearance: Dark Avengers #1 (2009). And yes, it was piloted by the former Green Goblin, Norman Osborn, who utilized confiscated Iron Man tech during his tenure as director of SHIELD. Yup.