When we're introduced to the Stargate, it's in the early-mid 90s, so them needing a big, bulky computer system would make sense, but as the show progresses, we see Tau'ri computer technology develop, either conventionally in the form of laptops like what the Atlantis team use, or computer crystals like what they fitted onto their starships.

Through it all, however, the SGC continues to use the same computer with comparatively dated hardware. Why keep it, instead of upgrading it to something more modern? Especially since one of the main issues that the SGC kept facing was that their dialling computer was not sophisticated enough to respond to some of the status codes put out by the stargate, causing all kinds of unpredictable behaviour.