As suitable means appropriate for the purpose, you can imagine that sneakers are more suitable than high heels at a track meet.
While suitable always means appropriate for a certain person or situation, sometimes it is not based on fact but merely a matter of opinion. A bicycle is not a suitable mode of transportation during a winter storm because the rider is exposed to the elements. However, the clothing that you think is just fine for school might seem unsuitable to the principal. Unfortunately, he has the final say in that one!