Why are there so many color spaces available?
There are a number of reasons why there are so many color spaces:


Color resolution - sRGB and similar color spaces only cover a portion of the visible spectrum. This means that they can be represented with fewer bits per color channel. (8 in the case of sRGB.) For something like Rec. 2020 or ACEScg, you need 16 or more bits per channel. If you don't have enough bits for each color channel, you'll start to notice banding in displays of gradations of color in the color space. If your hardware is limited, you'll use one that requires fewer resources, whereas if output quality is of the utmost importance, you'll use one that can represent more colors.
Output Devices - televisions and until recently computer monitors are only able to display a portion of the visible spectrum. (Likewise with printers.) If the output device can't display it, what's the point of being able to store and transmit it? Color spaces used for televisions and earlier computers were chosen based on what they could produce. Recently we've started to see wide gamut and HDR displays that can display a much larger portion of the visible spectrum (but still not all of it), so we're starting to see new color spaces for these devices.
Proprietary Needs - various vendors have created color spaces that match the devices and software they sell. The vast majority of photographers and videographers are home users doing it for fun. They don't need high bit depths and wide color gamuts. But professionals do!
Color spaces can also be designed to have certain properties that others don't. For example, some color spaces separate the colors based on perceptual uniformity (colors that appear similar to humans are grouped together), whereas other color spaces separate the color based on display hardware (RGB color spaces are generally used for displays that have red, green, and blue sub-pixels). Depending on what you're using the colors for, you choose a color space that makes working with colors in that way easy.