I think it's because Snarl is solar powered and therefore may not be of use in space missions. This was the only excuse I could come up with that is nerd-worthy.
His unique design makes Snarl particularly useful in sunny, arid environments. The large golden plates protruding from his spinal assembly are solar collectors. Although he can operate without sunlight, solar energy can increase his strength tenfold and his endurance to a virtually limitless degree. A swipe of his tail can shatter a 20-foot concrete cube. His heavily armored hide makes him resistant to most missile fire.
Snarl's dependency on sunlight makes him extremely vulnerable to attack at night. Although he can use the same fuel the other Autobots use, he operates at only a fraction of his strength when he does. He is also very slow and his uncooperative behavior patterns sometimes hinder his effectivess in dangerous situations.
- Wikipedia's page on him