Analysis Bomb-disposal operators quite like letter bombs, or "postals" as they are known in the trade. A postal device – especially if it's in an actual letter, as opposed to a parcel – is usually not very powerful. Even unprotected civilians are seldom killed by postals, and an operator in full armour can feel fairly relaxed when dealing with one.
Another plus is that if the package is unopened it can be handled and moved about without worry. After all, if the thing could suffer the Royal Mail's tender mercies without exploding, it isn't going to mind being picked up and X-rayed.
Similarly, as it was sent by post, one can generally rule out a timed detonation. The bomb maker has no firm idea when his weapon will be delivered. Almost all postals, therefore, are "victim operated" – that is, booby-traps intended go off on opening.
This in turn means that postals are normally constructed by low-calibre bombers, often disgruntled loonies acting alone. If an organisation uses letter bombs, it will normally be a lightweight, flakey, unprofessional one – animal liberationists or suchlike.
Serious criminals/terrorists/noble-freedom-fighters are usually seeking to strike at high-profile or security-conscious people, and such targets rarely open their own mail.
Say you're considering the letter-bombing of a senior business executive or a government official of any rank. A little thought will tell you that the only person likely to be hurt is an anonymous clerical employee, probably fairly blameless even in your twisted bomb-maker's world view. Thus, the serious players tend not to bother, leaving the letter-bombing field to the loopier small-fry elements.
The current letter-bombing campaign in the UK certainly fits the profile. By using letter bombs the perpetrator is effectively saying: "I am a fringe wacko of some sort." Speculation that the campaign is an extreme case of road rage against motoring bureaucracy is likely correct.