I'm going to add to what scythian1950 said.
First of all, I'm going to assume, as scythian did, that you're talking about seemingly outdated chips on spacecraft, as NASA's got plenty of modern computing power on the ground in its research centers
As scythian mentioned before, spacecraft development is a factor contributing to the age of the chips used. Spacecraft avionics and electronics are developed in parallel with the rest of the spacecraft, so it's not possible for a designer to simply drop in the newest technology right before launch.
Another factor to consider is the development of the chips themselves. Except in a few very rare cases, once a satellite is launched into orbit, it is not possible for anyone to reach the satellite to perform mechanical maintenance on it, so the systems on board must be very robust. For this reason, satellites typically use versions of commercial chips that have been modified to withstand the extreme radiation environments in which spacecraft operate. However, due to the extreme amounts of development and testing required to design rad-hardened chips, the availability of rad-hardened chips tends to lag a few generations behind regular top-of-the-line commercial products.
Finally, power consumption and functionality are other things to consider when selecting electronics for spacecraft. A desktop Pentium chip may be nice for multi-tasking and for displaying YouTube video images in real time, but these are extra functionalities that are simply not needed by a spacecraft, and they can come with a power cost in excess of 80 watts. In contrast, most spacecraft will make use of more specialized chips, such as DSPs and FPGAs, which may typically require less than 1 watt of electrical power.