I think it might be down to a simple difference between the aims of the IRA, ETA, etc, and the Islamist terrorists
They ( the IRA, etc ) wish to see X goverment out of area Y, or other limited aims. This requires them to deal with said goverment, when they have bombed the population into voting for people who'll give in. The Goverment involved is acting domestically. This is why they will sometimes issue warnings. If they can demonstrate power, and terrorise people without killing them, then they will seem more 'acceptable' to deal with later. If they start killing hundreds, or thousands at a time, rather than a few or a dozen, there is no way the public will accept any kind of deal with them.

From the BBC
Al-Qaeda's central goal is to act as a base, a kind of revolutionary vanguard, drawing as many as Muslims as possible into a broader jihadist tide of radical Islam.

This movement, they believe, would then drive the Americans from Muslim lands and overthrow governments like those of Pakistan and Saudi Arabia (which they see as propped up by the US) and lead the way for the unification of the umma - the Islamic community which would then follow what they see as a "pure" brand of Islam.

This allows no compromise, and so there is no need to ever talk to the goverments involved. The IRA have Sinn Feinn as a 'legitamte' front, but as far as I know, there is no eqivilent for islamic terrorists that target westerners. There are for groups operating in Isreal, and in North Africa, but not the ones supposedly threatening us.

The islamist terrorist groups seem more likley to kill more people, and be harder to stop. That is why the fuss is about them.

Bunnahabhain.