There’s one basic reason, and it should be taught in elementary school. Economic calculation is impossible under socialism. For economic infornation comes in the form of prices, and without some level of freedom to raise and lower price according to supply and demand there is no information. When prices are set by wild guesses or whimsy or political correctness, no one knows what’s going on.
That is exactly right. A related notion is that while there is no 100% perfect metric for determining how much someone "needs" something, a very good metric is how much they would be willing to pay to receive it, or how much they would demand in exchange for doing without it. If someone would rather have something than $500 cash, and someone else would rather have $250 cash than have the thing in question, that's a good sign that the first person "needs" the thing more. Even if the first person was sufficiently wealthy that $500 would not really affect him, and the second person was so poor that giving up $250 would pose a major hardship, giving the thing to the second person would leave both people worse off than e.g. having the first person receive the thing but give the second person $300.
Markets have a very strong affinity for Pareto-optimal allocations of resources (meaning allocations such that any small change which would leave anyone better off would have to leave someone else worse off). If economic efficiency would be enhanced by having market participants do something, odds are good that the market will do it if allowed to do so. Further, if market participants aren't doing something that would supposedly improve economic efficiency, that's a pretty good sign that they've judged that it won't.
There are some cases where pushing market participants to do something they wouldn't otherwise do may leave everyone better off. This sort of situation can arise in situations with positive network externalities, such that as the number of people doing something increases, the benefit to each person from doing so likewise increases. If few people were to do the thing in question, their benefit for doing so wouldn't be worth the cost. If, however, many people were to do it, the benefit could far exceed the cost. Temporarily artificially encouraging that action until the benefits exceed the cost may leave everyone better off than if there was no push to get the market "over the hump".
Unfortunately, politicians who want the government to intervene in the marketplace are almost never called upon to explain why their particular proposed intervention would direct the market toward an optimal resource allocation which it would not otherwise find. Instead, nearly all market interventions serve to--on an ongoing basis--prevent markets from acting to improve their efficiency. For example, programs which subsidize certain people's purchases of a certain commodity will reduce the quantity of that commodity available to non-favored people. To ensure that the now-scarcer resources are allocated efficiently, the price will increase, thus making the commodity less affordable to many people (contrary to the stated purpose of the subsidy, which was supposed to make it more affordable). If eligibility for the subsidy is expanded to accommodate those people, that will push the price higher, making it even less affordable for some people.
Unfortunately, many people seem to think the markets' reactions to such programs can't be predicted. That is, of course, nonsense. If a total of 5,000 units of some commodity are going to be produced per year, and that number is fixed, then people are going to be able to afford a total of about 5,000 units of that commodity in the absence of any program to subsidize select people's purchase thereof; that same 5,000 units will be "affordable" if the government subsidizes purchases by 10% of the population, or 50%, or 100%. The only things the subsidies will do are (1) mean that some people who would have been able to afford the goods won't be able to, because they've been bought by other people who didn't value them as much but were subsidized; (2) waste the public treasury's money. Supply-side subsidies might make more goods affordable, but would still represent an inexpenditure of public funds.