Why do people like David Irving deny the holocaust?

Most Holocaust deniers are anti-Semites of one stripe or another. They don't like Jews and sympathize to some extent with the Germans in WWII so they want to minimize, or even explain away completely, the Holocaust. In the Middle East, Holocaust denial is mostly about opposition to Israel. The memory of the Holocaust is something which Israel still uses to justify its commitment to Israeli security, including the occupation of the West Bank and the blockade of Gaza. For some Muslim opponents of Israel and it's occupation of Palestinian territories, denying the Holocaust is about trying to undercut a moral underpinning of the pro-Israeli argument. It also plays into a widespread perception in the Middle East of Israelis, and by extension Jews, as duplicitous and aggressive. If the Israelis could deprive Palestinians of their freedom for fifty years then why couldn't they invent something like the Holocaust to try and make themsleves look like victims?

Because they are ignorant bigots.

because it's not real and if it was real then jews deserved it.