Because AQ and many Islamists consider the Saudi royal family as corrupt (no question that is true), illegitimate rulers that rob Muslims of oil wealth by selling it cheap to the West in exchange for Western and US support financial and military for the regime. The real aim of AQ (at least according to its many public statements over the last 10-15 years) is to topple what they consider to be corrupt and illegitimate regimes in Saudi Arabia, Jordan, Egypt, Kuwait, etc....one of the key steps in accomplishing this is to force the US from the region.
The American people need to evaluate whether US support for these regimes and 50+ years of US and British involvement in the region (toppling some regimes, propping up others in an effort to control the region, to a large extent, and ensure that there are always pro-Western governments) is now worth it. That is why Ron Paul should be applauded...if nothing else, he is the only one reminding the American people that there is a cost to American government interventionism in the region (the ever-present threat of terrorist attacks against the American people). You may not agree with Ron Paul that the cost of US government interventionism in the middle east is not worth the benefits it brings the American people...but, man, how does anyone have anything other than contempt for the rest of the candidates who think that the American people are idiots who can be persuaded that there is no connection between the 50+ years of US government interventionism in the middle east and the threat against the American people...that the terrorists have targeted the American people because of how good and free we are?
As part of its global power position, the United States is called upon frequently to respond to international causes and and deploy forces around the world. America's position in the world invites attack simply because of its presence. Historical data show a strong correlation between U.S. involvement in international situations and an increase in terrorist attacks against the United States
--The Defense Science Board 1997 Summer Study Task Force on DoD Responses to Transnational Threats
In other words it didn’t make any difference that our forces were in Saudi Arabia and what Ron Paul said was al-Qaeda propaganda.
Al-Qaeda, and most Saudis want a Wahabi caliphate in Saudi Arabia. You know, like the Taliban, is that what Ron Paul wants?