And Noah and his sons and his wife and his sons’ wives entered the ark to escape the waters of the flood. Pairs of clean and unclean animals, of birds and of all creatures that move along the ground, male and female, came to Noah and entered the ark, as God had commanded Noah. Genesis 7:7-9

SPACE REASONS - there wasn't a lot of room on the ark for multiple sets of many animals.

MATING - you only need one male and one female to begin repopulation.

FAITH - God took the entire species down to only two animals.  That's the bare minimum.  We would want many in case there were any mistakes or mishaps.  God was saying "Trust me.  No need to worry about the margin of error.  I'll take care of you."