Liberals were one of the key political parties of the day and just a few years ago, formed both their own and coalition governments. In fact, they ran the government during the Great War and Churchill was a liberal then. Why weren’t they given prominent cabinet positions. Even Labour, which was a relatively new and until recently a small party, was given incredibly high ranking positions. Attlee was made Deputy PM even though many, even in his own party, saw him more as a peacekeeper. Sinclair had both experience as a military aide and an officer, yet he wasn’t included in any high ranking cabinet meetings. Why were the Liberals so marginalized in the Churchill Coalition?
Though the Liberal Party provided both First World War Prime Ministers, Asquith and Llloyd Gorge, the transition was hardly smooth, Asquith effectively heading the Opposition to Lloyd George with the Liberal Party split between the two. The party reunited to some degree for the 1923 General Election, but by that point had been eclipsed by Labour as the largest opposition party and lost further ground over the decade.
In the confusing series of National Governments of the 1930s the party split again into 'Simonite' and 'Samuelite' factions under John Simon and Herbert Samuel respectively; the former became Liberal Nationals (later National Liberals), the latter remained the 'official' Liberal Party. The latter fared poorly in the 1935 General Election, securing only 6.7% of the vote and 21 MPs, Samuel himself losing his seat (hence Sinclair becoming leader). The Liberal Nationals actually had an even smaller share of the vote (3.7%), but returned more MPs (33) thanks to targeting specific seats, and held cabinet posts under Baldwin and Chamberlain.
The National Liberals suffered from their association with Chamberlain, Simon being singled out as one of the titular 'Guilty Men' of the 1940 tract, and were shuffled away from key posts in Churchill's government, but Sinclair's Liberals were a fraction of the size of the Labour Party, hence not warranting a position in the War Cabinet, though Sinclair was made Secretary of State for Air.