In an answer, volume. Having worked in IT my entire professional life, I can safely say that a lot of the product being sold, either hard or a service such as internet access, is low margin.
Service industries such as ISP's make their money on Corporate customers. Residential have razor-thin to no margin on them. Once you've paid Telstra for space in an exchange for your DSLAM and your backhaul or a wholesale service and AGVC (backhaul for layer 2 DSL wholesale) you're then left with office costs, support, staffing, credit card charges, server room costs, high power bills and the most expensive of all, bandwidth.
If you want to talk about hardware having razor thing margins, look at MSY or CPL in Melbourne. They operate on massive turnover with a very very low percentage margin. Apple resellers have it worse. They don't even typically make 3-5% on Apple hardware sales, instead relying on third party accessories and extended warranty programs to make their cash. This is the reason that just about every Apple reseller will have a service department attached to it; it's one of the few ways they actually make a buck out of selling it (added to the fact that the Apple reseller agreement is so vicious, they require you to have $xx_,xx_x of stock on hand, and if something is end-of-lifed or superseded, you have to get rid of it within a certain timeframe, and you are required to purchase at least one item from every product group they sell for display).
IT is probably the worst industry for profit margin, right behind Aviation. What's the saying? What's the quickest way to make a small fortune? Start an airline with a large fortune