Because David Irving is a Holocaust denier, which is a pretty hefty reason to not be considered a historian anymore.

Irivng's 1977 book Hitler's War was a first foray into hisotrical bullshit territory when he tried to pin the Second World War on the Allied leadership and especially Winston Churchill but his reputation went completely down the drain when in the 80s when he really started to deny the Holocaust. First in Rommel biogrpahy he labeled the July 20 plotters as traitors but he got really serious about the denial in the early 80s when he attended conferences by the denialist IHR and was involved in the defense of Ernst Zündel in his holocaust denial trial. During this trial he tried to show that Hitler had no idea that the Holocaust was happening and after reading the Leuchter Report started to espouse that the Holocaust never happened.

In the early 90s he was a regular guest at Neo-Nazi gatherings in Germany denying the Holocaust using historical bullshit arguments and claiming he could all prove it with documents he had found in Poland buried under a tree (nobody ever saw these documents). He also held such speeches in Austria which lead to Austrian authorities issuing an arrest warrant for him, which was executed in 2005.

In 1996 he filed a libel suit against Debroah Lipstadt for writing he was a Holocaust Denier. During this case Irving tried to historically proof his theories that the Holocaust never happened but failed to do so. Several expert witnesses, among them Richard Evans, Christopher Browning and Jan van Pelt tore his theories to shred and debunked every one of his historical assertions.

In the words of Christopher Browning:

Not one of [Irving's] books, speeches or articles, not one paragraph, not one sentence in any of them, can be taken on trust as an accurate representation of its historical subject. All of them are completely worthless as history, because Irving cannot be trusted anywhere, in any of them, to give a reliable account of what he is talking or writing about. ... if we mean by historian someone who is concerned to discover the truth about the past, and to give as accurate a representation of it as possible, then Irving is not a historian.

In short Irving is not seen as a historian because he is a Holocaust denier, a liar, a Neo-Nazi, and someone who uses false evidence to prove something false that has countless times been proven as true. Denying the Holocaust is not something historians do and everyone who does it is not a historian because it goes against all standards of the craft. Simple as that.