Could anyone please help me to understand why Inventories are measured at “Lower of cost or net realizable value” under IFRS and “lower of cost or market value” under US GAAP?

The correct terminology is using “and” rather than “or” with respect to Lower of Cost AND Net Realisable Value (LCNRV)

BOTH IFRS and US GAAP apply LCNRV to FIFO and average cost.
US GAAP applies Lower of Cost OR Market (LCM) to LIFO and retail-method.