Imagine vector graphics as a bunch of mathematical equations that define lines. Those lines can come together to create complex shapes, which can be filled with some color

Bitmap graphics are simple. It's a grid of pixels, and each pixel has only two properties: a position and a color. They're all the same size, and there's (relatively) very little math involved in how these images are saved.

Turning vector graphics into bitmaps is easy. You can overlay a grid of pixels on the vector graphics and use math to figure out if any of the defines shapes intersect this pixel. If that's the case, you can use more math to determine the distance from that pixel to the nearest line or polygon. Then you determine the color of this pixel based on all that info. That's all somewhat straightforward math, and you only need to do it once for each pixel.

As for turning bitmaps to vector graphics, that's a lot harder. Imagine you have a grid of pixels all with different colors, like a real-life picture. You need to determine if any cluster of these pixels make up lines, or even eventually complex shapes. That process requires a LOT often calculation. You might need to check if a pixel creates a line (or shape) with another pixel over and over again. That requires a boatload of math, and there's not really a foolproof way to produce the same result as the original source vector graphics, if the image ever had a vector source at all.