Out-of-universe, in most sci-fi, the author is creating a "snapshot" of human(oid) society and technological progress in order to tell their story. This becomes especially important in film, as the level of technology is a primary determinant of the production design, which determines the "look and feel" of the film and others in a series. Star Wars, though conceived as a "trilogy of trilogies", was originally brought to the screen as just such a snapshot. The events of the entire original trilogy only cover something like 3 or 4 years. The prequel trilogy covers a span of roughly 11 years (with a 10-year gap between Ep 1 and 2, and a few months or so between 2 and 3), and there's a 20-year gap in between as Luke and Leia grow up and the Death Star is under construction. By the end of RotJ, we are very familiar with what ships in the universe look like, what people wear, etc etc.
While we as the human race have seen a quantum leap in technology in the last 35 years, consider, maybe, a random span of 35 years in the late 1700s. There was technological achievement, but the pace of it was snail-like compared to what we're experiencing now. Fashion and artistic tastes would have had more variance over such a time than technological advancement.
Now, somewhat back in-universe, fast-forward to a time in which humans have interstellar travel, energy weapons and energy shields, designer genetic modification, and at least pre-sentient algorithms (self-aware, though unable to imagine and invent which indicate true sentience). Such a time would be long past the collective "technological adolescence" that we in the real world are experiencing, and so the pace of technological advancement would again be relatively slow.
Also, prior to the events of the prequel trilogy, the Galactic Republic has reached a very bloated steady-state. While we see only a few major players and planets in the main storyline, several conversations infer that there are THOUSANDS of individual member states within the Republic, comprising MILLIONS of star systems. This brings up two important points: first, attempting to equalize the distribution of the fruits of this advancement is near-impossible. Various cultures have various ways of building things that have worked for them, and advancements in one field may be incompatible with another culture's technology. Second, with the size of the Republic, the number of people working to prevent the acquisition of new technology by their rivals is staggering. If you were to develop the Next Big Thing, you could practically guarantee that someone you didn't want to have it would either steal it or destroy it because they didn't want YOU having it. Think of GPS, stealth technology, nuclear weapons. The nations of Earth are all battling tooth and nail to both develop them and keep them out of others' hands.
Despite that, we do see technological achievement during the period of the movies. The Death Star is probably the biggest technological achievement in galactic history, and look what happened to it, because another faction didn't want it to exist. During the prequel movies, we see ship designs advance and new designs get introduced; by Ep 3, the universe has morphed from what we see in Ep 1 to what we're used to in Ep 4-6. Padme's hair looks very Leia-esque, the ship Padme and Obi-Wan use to go to Mustafar is a dead ringer for the one captured in the opening scenes of Ep 4, and Seinar's and Incom's ship designs are starting to look very much like TIEs and X-Wings.