Why imperial coastal forts remained obsolete

Why were old imperial coastal forts not upgraded with modern guns and machinery in the late 1920s?

Because Britain shifted the cost to colonial governments, and both London and local administrations resisted paying for the work.

Britain's failure to modernize older imperial coastal batteries came from finance and divided responsibility rather than a technical barrier. London expected local colonial governments to bear much of the expense, while those governments were even less willing to fund new machinery and armament. Neither level made the work a priority. The result was a predictable readiness gap produced by assigning responsibility without supplying money.