The reason is that the term "holocaust" can mean different things, and people include/exclude different groups and causes of death when making death estimates. When capitalised, The Holocaust refers to Jewish deaths - see the Oxford Dictionary definition 1.1. Here are a few comments on terminology

• When was the first time that the Holocaust was referred to as the Holocaust?

• Holocaust numbers 15-20 million

• Why is the Holocaust associated more with Jews than the other victims?

• Jewish vs. non-Jewish experience in WWII camps + why the Poles/Slavs aren't discussed as much?

• edit: here's another My old dictionary has only one listing for holocaust, as a common noun, lower-case "h". New dictionaries also list Holocaust, proper noun, upper case "H". What's the story behind this change?

More on counts if you're interested

• How many Jewish people were actually killed during the Holocaust?