why is Alfred Russel Wallace less known then Charles Darwin?

There are a few reasons.

Darwin seized the priority and ran with it. The Wallace-Darwin affair is a complicated episode in the history of science, but it basically went like this: Darwin studied and worked on his theory for decades, in secret, laboring over his "big book" that would prove evolution by natural selection to everyone. He told only a few trusted people what he was doing. On his own he probably never would have published it; he was too afraid of looking wrong to venture too public with it. He was, by this point, a well-respected member of the British scientific community for his previous book on the Beagle. Wallace was a relative unknown who was examining animals overseas when he came up with his theory. He wrote to Darwin as someone he wanted to get advice from, spelling out his idea. Darwin saw the similarities immediately and was distraught: he was going to get scooped. Darwin wrote to his confidants about it; they immediately suggested a way to preserve Darwin's priority while also giving credit to Wallace.

Darwin drew up a short announcement about the theory and signed both his and Wallace's names to it. This was presented at a local scientific society by Darwin's friends. Wallace was asked about this only after the fact (he was too far away to be consulted in "real time"). Wallace said he was fine with the arrangement — after all, his name was still on it, and he was now being attached to some scientific heavyweights. But it's also possible to see this as being exploitative on Darwin and his friends' part; they took advantage of the fact that Wallace couldn't say no, either practically (because they did it without telling him) or professionally (because he was in a far inferior professional position). Arguably Wallace got a bit gypped here.

Anyway, Darwin then rushed out a summary version of his "big book," which was On the Origin of Species. This laid out the basic theory of natural selection and evolution, and carefully skirted around some of the areas of controversy (it doesn't discuss human evolution, for example). This book catapulted the theory into the center of scientific debate almost overnight, and became the focal point for all future discussions about natural selection and evolution. Hence Darwin became the main person associated with all of this, even though Wallace was involved in a key way (spurring Darwin to publish; note that Darwin's "big book" never actually got written).

And Darwin really ran with it. Most people only know about Darwin's Origin of Species but he wrote a lot of other books as well about evolution. Descent of Man (which along with discussing human evolution, also discussed sexual selection), Expression of Emotion, and Variation of Animals and Plants under Domestication were all big books, putting out theories of evolution, cultural development, heredity, psychology, you name it. Darwin was very prolific, and his books had impact. Wallace wrote a few books but none of them had the same "punch" to them. This doesn't mean Wallace shouldn't be credited as an important scientist, but he is easily overshadowed by Darwin even if you ignore the other ways in which Darwin got attention first. Darwin's work (not just his ideas, but his actual work) is still directly engaged-with today by scientists; you can't say the same of almost anyone else from the late-19th century. He's part of a very small number of people from his time whose original work is still seen as relevant today.

But also: Wallace got weird. Wallace disagreed with Darwin over a few key aspects of the theory, which wouldn't be an issue by itself, but later in life he became a Spiritualist who believed you could talk to ghosts, became an anti-vaxxer (!), and several other dubious movements (phrenology, etc.). All of that meant that while Darwin was being turned into a "scientific saint" by other scientists (both while he was alive, and definitely after he died), Wallace was becoming the kind of guy that other scientists didn't want to entirely associate with. This isn't to say he was ostracized; he wasn't. But he was definitely treated with a wariness. The result is that after his death, you don't have people lining up to talk about the "great scientist Wallace," whereas you do have them doing that for Darwin.

(No one asks "What Would Wallace Do?", but they do for Darwin. Even though the answer for almost everything in Darwin's post-Beagle life would be: "stay home and worry about it to the point of puking." But this is the difference between a scientific symbol and the real person!)

And posthumously, Darwin got "reinvented." The history of modern evolutionary theory is a bit more round-about than most people know. Basically there was a time between Darwin's death and the 1930s which has been called the "Eclipse of Darwinism," where Darwin was considered important but not actually correct. It's a long thing to explain here, but the basics is that wasn't at all clear how to reconcile several key elements of Darwinism (gradualism in particular) with what was being learned about how heredity worked (something Darwin himself was pretty much ignorant of). It wasn't that people didn't think Darwin was right that life had evolved, it's that they weren't sure gradualist natural selection actually explained how it worked. Anyway, by the 1930s sufficient understanding of how genetics worked in populations to attempt a comprehensive, genetic discussion of evolution, and this basically "redeemed" a version of natural selection. This "modern evolutionary synthesis," as its creators called it, was explicitly linked back to Darwin's work specifically. So this really cemented Darwin as not just another scientist of his time, but a special kind of scientist whose work really transcended its era and even the science of its time. Again, it's not that they didn't do this for Wallace (they didn't), it's that they didn't do this for anyone but Darwin; Darwin is super anomalous in this respect. (You see something similar, incidentally, with the "rediscovery" of Mendelism as well, where the figure of Gregor Mendel was plucked out of his original context and modified for a later context.)

We might ask: Should Wallace get more credit? I mean, it depends on what you think the function of credit is. It certainly isn't merely "who thought of an idea first," since that is a pretty complicated thing and most people only know a tiny tiny tiny number of scientists who worked on a given concept. (And it's not like ideas are ever "truly new" anyway — they are always building on previous ideas, something Darwin would have readily admitted.) Credit serves a social function, and scientific credit is a way for scientists to talk about both the progress of science and the ethos of science. So it doesn't bother me if in biology class nobody talks about Wallace. They don't really talk about Darwin, "the man," in such classes, either. "Darwin" is serving as a partially-true stand-in for talking about how science works in such a context. Sometimes Wallace is discussed in these contexts, because it makes for an interesting case-study in priority and inspiration (and a nice moral tale about the dangers of not publishing in a timely manner!), incidentally.

But if you're interested in the actual history of evolution — well, yes, you have to talk about Wallace, in detail. And many other figures as well. And such works of history do exactly this (for Darwin biographies, I am partial to the work of Janet Browne as well as Demond & Moore, and for a broader evolutionary history the work of Peter Bowler is the reference).