Why is stabilizer trim often controlled by electric motor while other surfaces are mostly hydraulically controlled?
Weight, cost, reliability and maintenance.

See ACTUATION SYSTEM DESIGN WITH ELECTRICALLY POWERED ACTUATORS

Safety

See THE A380 FLIGHT CONTROL ELECTROHYDROSTATIC ACTUATORS, ACHIEVEMENTS AND LESSONS LEARNT


  In terms of safety several aspects can be 
  highlighted: As far as power source redundancy 
  is concerned the number is increased from 3 to 4 
  since 2 electrical systems replace 1 hydraulic 
  system. Furthermore an additional margin of 
  safety results from the introduction of the 
  hydraulic/electric dissimilarity in the power 
  sources: This provides further protection against 
  common failures, such as maintenance errors, 
  which may affect all the hydraulic systems, 
  whatever their number. Moreover the electrical 
  power provides flexibility in routing, resulting 
  in an easier segregation of power distribution 
  routes against engine burst and other “particular 
  risks”, and an isolation and reconfiguration 
  capability that hydraulic systems cannot offer.
  
  The reduction in the total number of hydraulic 
  components results in improvements of the 
  MTBF and dispatch reliability, by elimination 
  of potential leakage sources. 
  
  The elimination of the generation and 
  distribution components associated to one of the 
  hydraulic systems (pump, reservoir, filters, 
  plumbing…) and the replacement of the 
  associated hydraulic actuators by electrically 
  powered actuators results in weight savings.