Postal voting has many problems:
open to abuse
means some people vote well ahead of the rest of the population, while campaiging etc is going on.
damages the feeling of commuity by partaking an a vital 4-5 yearly ritual for about 15 mins.
Online voting has same problem of:
loss of community
but even more importantly removes any certainty of actual votes and would make it much too easy for inaccurate (either intentional or accidental) results to be achieved. Paper leaves a fantastic easily visible accuracy which is very hard to manipulate.