This is a guess.
Tryptophan is a bulky amino acid. Having a lot of if would make the protein bulky. It is anyways hydrophobic and cannot be present on protein exteriors; high tryptophan may distort hydrophobic core possibly because of its bulkiness and ability to form pi-stacking interactions.
However, tryptophan is an essential amino acid and cannot be made de novo in humans. Indeed, it has only one codon perhaps because of its limited usage.
Nonetheless, tryptophan, despite its low abundance, is important in stabilization of protein structures such as β-hairpin  and it is also required for synthesizing serotonin.
Leucine is present in the hydrophobic core and β-regions. It also surrounds active sites which causes exclusion of water; this in turn facilitates catalysis by allowing polar residues to bind to the substrates .
I don't have any comment on why not isoleucine or valine instead of leucine.