When I was flying easy's B737s they were all CAT111A with a decision height of 50ft and a RVR of 200m, unless, for technical, reasons the aircraft was temporarily downgraded. The diversion policy is that a nominated alternate must be operating in conditions which would allow an approach one category higher than minimum. For example, if the airfield, crew or aircraft were limited to CAT1, then the weather conditions must be good enough for a non-precision approach. And these conditions must be forecast to exist for one hour either side of the ETA at the destination. I suspect that an immediate diversion to PIK looked the safest course of action. Easyjet may be low cost, but it's never been low safety (except for exhausted crews!)
The company claims it saves tens of millions of pounds a year by discouraging the carrying of holding fuel - when conditions are allow- so they are happy to absorb the costs of occasional diverts. These savings help keep air fares low. If the wx at Aberdeen was forecast to be poor then the policy is to take holding fuel; I assume the crew weren 't, or felt the wx wouldn't improve - only they will know.
JB