Final answer:
Underwood Typewriters lost their market leadership in 1923 due to competition from other typewriter manufacturers and the emergence of more efficient typesetting machines.
Step-by-step explanation:
The primary reason why Underwood Typewriters lost their market leadership that they established in 1923 was due to competition from other typewriter manufacturers, particularly the Remington Typewriter Company, which introduced their typewriter on the market.
Additionally, the automation of typesetting with machines such as the Monotype and Linotype machines also contributed to the decline in demand for typewriters. These machines were able to cast and set type much faster than typewriters, making them more efficient for printing purposes.