The three most common arguments for this are:
Free trade prevents the growth of domestic industries. When free trade is practiced, countries that do not yet have much industry cannot build industries. Their “infant industries” can never compete with established firms in developed countries.
Free trade agreements prevent governments from protecting their workers. These agreements can prevent governments from putting in place stringent rules to protect worker rights and provide them with good working conditions. They can prevent countries from having adequate environmental regulations as well.
Free trade hurts agriculture in developing countries. According to this link, Mexico lost 1.3 billion agricultural jobs in the 10 years after NAFTA was signed. This is because developed countries’ agricultural sectors are much more efficient and can generally outcompete farmers in developing countries.