Why sometimes for Aluminum Electrolytic Capacitors Impendence is mentioned not ESR?
ESR and similarly ESL would let you calculate the entire impedance curve of the capacitor, however it takes longer to test components to make sure they are within that spec, 

If your company is only selling to customers who care about impedance at 100KHz and 1MHz, you only test to those specifications, in reality the lower frequency values can usually let you reverse out an ESR range, 

Impedance is the square root of (Xc2 + Xl2 + R2), so with a low enough frequency, XL does not have enough significance, leaving mostly R or your resistance,