Background: Orchestral Manoeuvres in the Dark (OMD) are an English electronic music band formed in Wirral, Merseyside in 1978. Spawned by earlier group The Id, the outfit is composed of co-founders Andy McCluskey (vocals, bass guitar) and Paul Humphreys (keyboards, vocals), along with Martin Cooper (various instruments) and Stuart Kershaw (drums); McCluskey is the only constant member. OMD released their debut single, "Electricity", in 1979, and gained popularity throughout Europe with the 1980 anti-war song "Enola Gay".
Context: During 1988 the band appeared poised to consolidate their US success, with a support slot for Depeche Mode's Music for the Masses Tour at the Rose Bowl in Pasadena, California on 18 June 1988, a top 20 US hit with "Dreaming" and a successful "Best of" album. However, it was at this point when OMD broke in two. Graham and Neil Weir left at the end of the 1988 US tour and co-founder Paul Humphreys subsequently called it a day, unhappy with the band's commercial orientation. Finally, Cooper and Holmes left OMD to join Humphreys in founding a new band called The Listening Pool in 1989. This left only McCluskey to carry on, essentially becoming a solo artist working under the OMD banner. McCluskey's first album from the new OMD was the dance-pop Sugar Tax LP in May 1991, which charted at No. 3 in the UK. McCluskey recruited in 1989 Liverpool musicians Raw Unlimited (aka Lloyd Massett, Stuart Kershaw and Nathalie Loates) as collaborators for the making of Sugar Tax: writing credits carefully distinguished between songs written by OMD (i.e., McCluskey) and songs written by OMD/Kershaw/Massett. This iteration of the group was initially successful with hits like "Sailing on the Seven Seas" and "Pandora's Box", with lesser success on fellow chart entries, "Call My Name" and "Then You Turn Away". McCluskey's live band was then formed by Nigel Ipinson (keyboards), Phil Coxon (keyboards) and Abe Juckes (drums) since late 1990. Kershaw, Ipinson and Coxon from 1992 to 1996 contributed to OMD albums and a 1993 tour. The fifth track from Liberator (1993), "Dream of Me", was built around a sample from "Love's Theme" by Love Unlimited Orchestra, a track which was written and produced by Barry White. To release the "Dream of Me" track as an OMD single, however, McCluskey had to agree that the single release of the track would remove the actual "Love's Theme" sample, but still be officially titled "Dream of Me (Based on Love's Theme)", and furthermore would still give a writing credit to White. Paul Humphreys, while no longer part of the group, co-wrote the single "Everyday". Also in 1993, McCluskey made contributions to the Esperanto album, a project by former Kraftwerk member Karl Bartos (then working under the moniker of Elektric Music). McCluskey returned with a rotating cast of musicians for the 1996 album Universal, which featured two songs co-written by Humphreys as well as a holdover from the Esperanto sessions, co-penned by Bartos. The record spawned OMD's first Top 20 hit in five years, "Walking on the Milky Way".
Question: Why did they split?

Answer:
Paul Humphreys subsequently called it a day, unhappy with the band's commercial orientation.