Fox did it to make the show look more popular.
The, alas now defunct, Sliders: Dimensions of Continuity website (usually a highly reliable source of information) indicated that the air-dates were messed around with at the insistence of the network.
Why Did Co-Creator Tracy Tormé Leave Sliders?
Even from the start, Fox gave co-creator Tracy Tormé many problems.
They refused to show season one in its intended order...even when the
episodes linked together. Fox refused to allow the series to be an
open medium...and constantly tried to place it in a single box (they
chose action/adventure). This hindered Tormé a great bit. Fox also
constantly restrained Tormé on any of his ideas...this even went to
the point that Fox almost didn't let Tormé resolve the season one
cliffhanger. In fact, the season two premiere "Into the Mystic" was
filmed with no resolution...and after it was finished, Fox gave
permission and Tormé had to go back and make changes as he could to
resolve the season one cliffhanger.
Behind the Scenes - Sliders: Ultimate FAQ
We learn from a SliderWeb interview with Season 3, 4, 5 director David Peckinpah that the main aim in re-ordering the episodes was to game the ratings, making the show look more popular by airing blockbuster episodes in the "Sweeps" period when audience numbers were collected.
On why we never saw Quinn killing any Kromaggs before…
"I felt episode six was more important to air during sweeps than episode five. Episode five, 'Quinn Kills a Bunch of Nazi Kromaggs
While Maggie Disguises Herself as a Stripper,' just didn't seem to
have that special something that this episode had."
Earth 5260: The Peckinpah Interviews
and in case you were thinking that original showrunner Tracy Tormé was entirely unblemished in all of this, it turns out that he also had plans to muck around with the airdates for much the same reasons.
Even after Sliders came back in its second season, though, it walked
the ratings borderline. As May sweeps approached, Tracy pushed to air
what he thought was the season’s best episode, Post Traumatic Slide
Syndrome, an episode with a controversially ambiguous ending and few
special effects. The network vetoed Tormé and slid in instead an
episode featuring dinosaurs, complete with a “Jurassic Park”-ish ad
“I saw the promotion and I thought, ‘Oh, Christ,’ ’cause we can’t do
Jurassic Park, and we can’t do Twister like the movies can do it. To
me, that’s worrisome.” However, the dinosaur episode, In Dino Veritas,
aired to the season’s best ratings, and Tormé isn’t afraid to admit he
“It worked,” he says. “That’s probably one of the reasons we came back
for another season; the ratings on that episode were that good. I’ll
give the devil its due.”
THE UNIVERSE INTERVIEW: TRACY TORMÉ