I have heard that recently researchers have developed white laser light by combining semiconductor laser sources of different colors. However, I have heard that before these lasers become a practical reality they must be powered by electricity and not other lasers. Why is this the case?
The semiconductor lasers that are used to produce white light are powered by electricity, so that solves your problem.
As lasers emit monochromatic light (1 single frequency or colour), they cannot be white. To get white laser light, you start with a blue laser. This blue light is directed at some phosphorescent material (a "phosphor") which, when activated by the blue light, emits light of a different colour, e.g. yellow. If you use several different phosphors you can get a mixture of colours that looks white - white is an equal mix of red, green and blue.
White LEDs work in the same way. Lasers are just a much more intense source of light than LEDs. In both cases you start with blue light and convert it to white using phosphors.
White lasers are starting to be used in car headlights, e.g. by Audi and BMW. This website describes how BMW's laser headlights work. It also goes into some detail about why these lasers are not dangerous to the eye.
Each laser is made up of three main part: pump, amplifying medium and cavity. White laser is semiconductor laser; which means the amplifying medium is a semiconductor. In semiconductor lasers, pumping is electrical; which means the amplifying medium requires electrical power to be stimulated.
The semiconductor of the white laser is an alloy made of zinc, cadmium, sulfur, and selenium, and is nanometers thick. Three main-colors lasers are made by this alloy. When lasers of different colors are emitted and combine with each other and the result is a white light with laser-light properties.
Finally it must be said that: white laser is mixture of laser lights in all colors.
I think the original question is, why the white laser is electrical pump instead of optical pump? Because electrical pump is much easier for practical use. You just inject current then get light, especially you have multiple lasers with different colors integrated together.
Why they made white laser? Because the light can propagate much longer and have much higher power. They are definitely not for human eyes.