There aren't more terrorist attacks, because the classification as "terrorist" is a false classification. In reality the same mindset, motives, situation, that allegedly drive "terrorists" drive the people who are committing murders of their friends/family/coworkers/random-people every day. When an act of violence is carried out by a Muslim-named individual with an ostensibly political motive, you classify that as terrorism, whereas the underlying psychosis that leads to the crime is the same as most other acts of violence. The manifestation of that psychosis has circumstances and external justification that are irrational, random, and individual to each person.

So this question is like asking, "Why aren't there more 5'7", 11 shoe size, blue sweater-ed murderers?" The fact that there are not more "terrorist attacks" is proof that the classification of "terrorist attacks" is an unnaturally narrow one, based on arbitrary features of the perpetrators.