TTL (and therefore 74S, 74L, 74L LSTTL and 74HC CMOS) decoder outputs are typically active-low. The reason (which is the question in your title) is not completely obvious to me, but were I to speculate I might point out that it is somewhat arbitrary for normal push-pull outputs, and may be related to the way the ancients would "wire-or" open-collector outputs (eg. 7405 and 7438), implying an active-low.
If you're driving a 7-segment display, you probably want a BCD-7-segment decoder such as a 7447 or variant, or something from another family. The outputs of this chip are indeed active-low, so you'd connect the anode of a common-anode display to the supply voltage (+5 typically) and each output to a segment (through an individual current-limiting resistor, of course). Each "low" output results in the respective LED turning "on".
If you happen to have a common-cathode display, all is not lost, you can use a part such as 74HC4511, which has active-high outputs. I recommend this part, as it's likely to be around for quite a while longer.
You could also use the 74LS48 (link with 7447 above), which has active high outputs and internal 2K (nominal) resistors to directly drive a common-cathode display with no external resistors (brightness may vary too much to be acceptable between adjacent digits- it's very loosely guaranteed, so this is a bit risky, and the part may be hard to find).