Why does biomagnification has a larger impact on organisms at the top of
the food chain.

Biomagnifications largely affects the organisms at the highest trophic level.

Explanation:

The increase in concentration of toxic chemicals while moving up in a food chain is called biomagnification. What starts out as a small concentration at the lowest trophic level turns out to become a higher concentration  at higher trophic levels.

Thus it is the organisms at the highest trophic level that are the most affected.  This accumulation occurs because the organisms have not evolved enough to posess mechanisms that facilitate detoxification of various substances.

Such substances are called novel substances and DDT is  an example. Removal of substances like metals is also difficult. Thus  a harmful chemical component travels up various trophic levels increasing in concentration at each trophic level.