Clcok frequency becomes low means time period of clock becomes high. When this time period increses beyond the time period in which the non-volatile memory contents must be refreshed, we loose those contents. So, clock frequence can't go below this value.
The cache, register etc. inside CPU and RAM for primary memory are all examples of volatile memory and needs refresh within the specified time period or else will loose their contents.
Non-volatile memory are being developed and computers in future should be tailor made for that.