Why can't dual port RAM be read out using the Quartus In-System Memory Content Editor?
Per the In-System Modificationof Memory and Constants.


  When you specify that a memory or constant is run-time modifiable, the Quartus II software changes the default implementation. A single-port RAM is converted to a dual-port RAM, and a constant is implemented in registers instead of look-up tables (LUTs). These changes enable run-time modification without changing the functionality of your design.


Essentially in order for ISME to work, it requires one of the two memory ports in order to be able to access the contents of the RAM. 

Only single-port memorys can therefore be used with ISME, because if you use a dual-port RAM the second port is already in use by your design and is therefore not available to ISME.