Lithium hypochlorite is the only solid form of chlorine that when added to a pool does not have the side effects of increasing CYA or CH. The following are chemical facts independent of product concentration or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

So if you don't want to increase CYA nor CH but of course need to add chlorine regularly, then lithium hypochlorite is the only solid source of chlorine that will do that. The solid form has a long shelf-life compared to chlorinating liquid that must be turned over regularly (bleach lasts longer due to its lower concentration of chlorine, but takes even more space as a result). At 35% Available Chlorine, it is nearly 3 times more compact than chlorinating liquid and 4-5 times more compact than bleach. So from a product storage and space requirement, lithium hypochlorite fits better into a stores' "profit per square foot" model (Cal-Hypo, Dichlor and Trichlor are even more compact, but are less expensive per FC and per volume).

Marketing goes a LONG way in pool stores as does deceit. Pool stores often do not tell people about the buildup of CYA using Trichlor pucks/tabs or that higher CYA levels make chlorine less effective so algae can grow unless you proportionally raise the FC level. Instead, they sell people higher-margin products such as algaecides, phosphate removers, clarifiers, flocculants, enzymes, "shock", etc. It's not all the pool stores' fault as they often just pass along what they are told from the manufacturers of those products. The lie that "CYA doesn't matter; only FC matters" has been told for nearly 40 years even though the science was definitively determined in this 1974 paper.

There are pool stores that provide the full array of products including chlorinating liquid. There is one in my area where I buy 12.5% chlorinating liquid that they store in the front of the store in the shade and they sell a lot of it, but they also have pyramid stacks of Trichlor buckets inside the store as well.