What is the reason behind high resting membrane potential of pacemaker cells?
The pacemaker potential is interesting (to a biologist) as it involves your typical Na/K channels, Ca channel, as well as a funny current (If) or alternatively called hyperpolarization-activated current.


  The funny current is a mixed sodium-potassium current that activates
  upon hyperpolarization at voltages in the diastolic range (normally
  from -60/-70 mV to -40 mV). When at the end of a SA potential the
  membrane repolarizes below the If threshold (about -40/-50 mV), the
  funny current is activated and supplies inward current, which is
  responsible for starting the diastolic depolarization phase (DD); by
  this mechanism, the funny current controls the rate of spontaneous
  activity of sinoatrial myocytes, hence the cardiac rate.





  Pacemaker activity (or spontaneous electrical activity) of the
  sino-atrial node is based on the presence of a special phase called
  the diastolic depolarization during the action potential, in which
  cells depolarize spontaneously towards the AP threshold. Animal
  studies (mostly conducted on rabbit heart) have identified that this
  net inward current during the diastolic depolarization phase is the
  result of a complex interaction of multiple inwardly and outwardly
  directed ion currents....


Verkerk, Arie O., Antoni CG van Ginneken, and Ronald Wilders. "Pacemaker activity of the human sinoatrial node: role of the hyperpolarization-activated current, I f." International journal of cardiology 132.3 (2009): 318-336.

Also this


  The SAN is a complex tissue with regional differences in morphological
  and electrical properties.Animal studies have revealed that
  pacemaking in SAN cells follows from diastolic depolarization driven
  by a net inward current, which results from an interaction of multiple
  ion currents. Inward currents are activated during diastole:
  hyperpolarization-activated pacemaker current (If), background Na+
  current (Ib,Na), sustained inward current (Ist), T- and L-type Ca2+
  currents (ICa,T and ICa,L, respectively), and Ca2+-release activated
  Na+–Ca2+ exchange current (INCX). Conversely, outward currents are
  deactivated: rapid delayed rectifier K+ current (IKr) and slow delayed
  rectifier K+ current (IKs). The relative contributions of these
  currents to diastolic depolarization are a matter of debate.


Verkerk, Arie O., et al. "Pacemaker current (If) in the human sinoatrial node." European heart journal 28.20 (2007): 2472-2478.

Might I suggest you read up on the papers mentioned above?

You might also be interested in this:

Noma, Akinori. "Ionic mechanisms of the cardiac pacemaker potential." Japanese heart journal 37.5 (1996): 673-682.