It's a combination of factors but it's more of a spectrum than you suppose.

Look at ESB the hairstyles are less funky, the lighting is a little more sharp, Vader is a bit more polished and detailed and it feels less seventies more eighties but ROTJ is the only Star Wars film actually made in the eighties (not unless you count the Ewok films). 

I am assuming you are referring to the "look" of a film outside of things like fashion. This boils down to two factors: production design, and more importantly cinematography.

With production design, it's mainly just style and taste of the time. However, you have to account for materials, mainly plastics, that have only existed recently. When it comes to makeup--whether it's just regular makeup or prosthetic effects and stuff--the chemicals used have changed over the years, so films from different eras use different materials. Mold making, even the types of wood used to make sets have changed, so that it's easier to work with and easier to do complex designs.

Cinematography is a bit more obvious, and again comes down to style, but also technology. Things as simple and taken for granted as kino-flo flourescent lighting were only really introduced in the 1990s, and completely changed the "look" of a lot of films. You had the elimination of arc lights, the elimination of sound blimps making cameras lighter and more moveable, steadicam which really only came around at the end of the 1970s and wasn't commonly used until close to the end of the 1980s. Also film stock: the chemical composition of film stock is constantly changing--even today--so films from certain years are shot on the same two or three types of stock, which effects sharpness, grain level, contrast and colour. You also have matters of taste, so some years filters are in style and some years they are not. Some years handheld camerawork is more acceptable, and some years it is not. It comes and goes. It also depends on studio versus location, which are lit slightly differently and affect shot choice. Also, things you don't think about like cranes, dollys and wireless technology effect what types of shots cinematographers can do--for example, remote focus pulling is relatively recent, so it's only in the last 20 years that you could do really crazy shots because the operator and focus puller didn't have to be strapped beside the camera.

As to whether these can be replicated--you can, but it's a bit difficult. Technically it should be perfectly possible, but I think people who try to do so don't fully understand why films from a certain time look a certain way, so they only get it "mostly right."