Shivans aren't primitive, they are better than the GTVA at certain technological aspects (beams, subspace) and worse in others (stealth, fighter weaponry, support/re-arm/repair ships). How they make up for it is their attack-oriented designs + TOTALLY OVERWHELMING numbers, enough to challenge even a Zerg Overmind with full command of all its broods.
Due to the Shivans lack of challenge by any species we've seen so far, they haven't really had a need to use complex battle strategies or tactics to win, nor have they needed to quickly advance their technology. This is probably why the Shivans don't improve by leaps and bounds, unlike the GTVA.
The GTVA doesn't have anywhere close to the numbers the Shivans have, which forces them to use complex strategies/tactics to minimize casualties and maximize kills. This also forces them to quickly improve their technology so their numerically inferior forces stand a chance against the Shivan swarms.
Another way to put it is that, the Shivans don't really have much selective pressure to improve and survive, while the GTVA must CONSTANTLY adapt, grow and advance in any possible way just to fend of the Shivans.