They almost exclusively eat salmon. Humans have destroyed salmon populations. They share their territory with other natural prey like seals. They kill seals for fun. They can't figure out that seals are edible.

But I'm guessing that's not the main reason their population is declining

Pacific Northwest orca are among the most contaminated marine mammals in the world, due to the high levels of toxic anthropogenic chemicals that accumulate in their tissues.[115] Implicated in the decline of the southern resident orca population, these widespread contaminants pose a large problem for conservation efforts. While many chemicals can be found in the tissues of orca, the most common are the insecticide DDT, polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs).[116] Each of these have detrimental physiological effects on orca,[117] and can be found in such high concentrations in dead individuals that those individuals must be disposed of in hazardous waste sites.[118]

Correlative evidence shows orca may be vulnerable to effects of PCBs on many levels. Research has identified PCBs as being linked to restricting development of the reproductive system in orcas and dolphins.[119] High contamination levels leads to low pregnancy rates and high mortality in dolphins. Further effects include endocrine and immune system disruption, both systems being critical to mammalian health and survival.[117] A study examining 35 Northwest orcas found key genetic alterations that caused changes to normal physiological functions.[120] These genetic level interferences, combined with the varied effects of PCBs at other physiological levels, suggest these contaminants may be partially responsible for declines in orca populations.

https://en.wikipedia.org/wiki/Southern_resident_orcas