Why haven't the remaining TRSAs been converted to Class C airspace?
Class C airspace is established by a rulemaking procedure (i.e., Notice of Proposed Rulemaking, comments, publication, etc.).  TRSAs are not established by rulemaking action.

From FAA JO 7210.3Z- Facility Operations and Administration -para. 11-1-3 &amp; 4

Class C:


  Class  C  airspace  must  be  officially  designated  by
  airspace action in 14 CFR Part 71 and is established
  solely to define the airspace in which all aircraft are
  subject  to  operating  rules  and  equipment  requirements specified in 14 CFR Part 91.[emphasis is mine]


TRSA:


  TRSAs 
  are not officially designated 
  by airspace
  action and were established solely to define an area
  within which a separation service will be provided.
  [empasis is mine]


Within a TRSA the (vfr) pilot is not required to participate.  However, within Class C airspace a (vfr) pilot is required to participate.

It is not as simple as a charting issue:

If a Radar Facility Manager wants to change a TRSA and make it Class C airspace a staff study must be done and proper justification has to be documented and approved. See Class C Airspace Processing (staff study protocol) FAA Order 7400.2L - Procedures for Handling Airspace Matters - para: 16-3-1

Since the implementation of Regulatory Class C airspace requires more ATC involvement to provide the service (Class C service) and requires mandatory participation on the part of vfr pilots, the process can be a significant challenge that has to be clearly needed for the area in question and fully justified with consideration of public comments, etc., and a variety of other procedural steps to comply with rulemaking procedures.

In short, unless the Facility Manager (for that area) has determined that a higher level of air traffic control  is necessary for safety/service, etc., and then provides justification and considers comments from the public (who may not want the additional restrictions of Class C), changing a TRSA into Class C airspace won't happen.