Why Churchill remained the likely First Lord in 1939

Why would the government choose Churchill as First Lord of the Admiralty despite Gallipoli and doubts about his reliability?

Because the pool of experienced, publicly credible wartime candidates was very small, and Churchill sent the strongest signal of seriousness.

The apparent abundance of members of Parliament shrinks once age, experience, reputation, and competence for senior wartime office are considered. Further candidates are needed as foreign secretary, service ministers, or even prime minister, reducing the practical choice still more. The Admiralty appointment also had to reassure the British public and signal resolve abroad. Churchill's record included serious failures, but his fame and naval experience made him one of very few politically usable candidates. His return therefore appears as the product of constrained cabinet selection rather than a claim that he was flawless or uniquely trusted.