Becoming a good DM requires dedication and effort, but I wouldn't expect it to be "hard" for most people interested in being DMs. Some (or most) may struggle with one or more of the core DM skills, but can usually gain a modicum of proficiency given enough practice. There isn't any secret to being a great DM or hidden techniques that will elevate your games.
What are the core DMing skills? In my mind they are:
1. Leadership. As a DM you are running the game. All the players are looking to you to tell them what's going on, and to figure out what happens next. That can be a lot of pressure, even in a friend group. Some people are born leaders and revel in being at the center of everything. Others gain confidence over time. As players, it's important to support your DM and not heap undue pressure on them.
2. Rules Knowledge. As a DM, you should have a decent knowledge of the rules (at least the basics resolution systems). The more rules knowledge you have, the better, but comprehensive, encyclopedic knowledge of the game is not mandatory. If you don't know something, you can look it up (or have a player do it while keeping the game going). If a player uses a spell or power that you're not familiar with, have them read out what it does. Some people spend a lot of time reading rulebooks and have this stuff down pat. Others just can't remember the fine details of the game. As a player, don't expect the DM to know everything in the rules. Some have busy lives and are just doing their best.
3. Making Decisions. As a DM, you need to make decisions. Does this rule apply in this context or not? Should I say "yes" to the PCs' cockamamie plan or not? Hmm, I rolled up a dragon on the wandering monster chart, should I have it appear or not? Was the PCs' argument to the duke convincing or not? Should I coup de gras the downed PC or not? Often, there is no right or wrong answer. Everything is a big fat...eh..maybe... And that's where the challenge comes in. Choose right and the game is fun. Choose wrong and the game might be "ruined". Stressful! The good news is that individual decisions generally don't mean much in the agregate. Very rarely does a decision "ruin" a game. And, if it does, you can usually work with the players to retcon it. The importance is trying for consistency in your decision-making.
4. Imparting Information. This is a tricky one. How much information does the DM give the players? How might a chamber be described to indicate potential dangers or opportunities? How much backstory gives enough detail to be interesting but not so much the players get overwhelmed? How does one describe a serious threat that might result in a TPK? How does one indicate the "adventure path" to keep the PCs going in the right direction? What happens if the party absolutely refuses to take the hint? For novice DMs, running published adventures is helpful here as the well-written ones will usually have good answers to most of these questions. Otherwise, it's up to practice. I also advise completely breaking the fourth wall and telling the players outright if it appears some sort of massive misscommunication is going on.
5. Bringing the Fictional World to Life. This is the fun descriptive stuff and roleplaying that sets the scene. Some people are natural storytellers or Matt Mercer-types that really get into this aspect. Others struggle with it. As with anything, people get better with practice but this one is more about your DMing style. What feels comfortable for you. Do you like elaborate descriptions filled with hyper-specific details or poetic allusions or do you keep it terse and punchy? Once again, there isn't a best way here. Every good DM finds their voice in time. And speaking of voices, don't get hung up on perfectly replicating the voices of NPCs. Most DMs are not trained thespians and shouldn't be judged as such. Relax and have fun with it. If you don't have fun RPing NPCs, it is perfectly acceptable to say things like: "The duke thanks you for your service to the kingdom and rewards you with a chest filled with golden coins" rather than making a grand speech about it.
6. Small Group Dynamics. This is another tricky one, that requires a bit of maturity on the part of the DM (well, all players, but everyone usually looks to the DM to sort through this). This is how well your group gets along and how much everyone is having fun. It requires understanding what different players want out of the game, who has influence and who does not, who is hogging the spotlight and who is getting pushed out of it, how decisions are reached by the party, and even out-of-game scheduling, snack-bringing and other logistical concerns. Most campaigns fail because of scheduling or intra-player conflict. Managing players and groups becomes easier with greater emotional intelligence and more experience with your fellow players wherein proclivities are learned and trust is built. Some DMs are blessed with fantastically supportive groups. Others have to deal with problem-players, constant turnover, or general chaos. The good news is that life teaches one how to deal with this stuff and, better yet, DMing can help prepare you for life. How cool is that?
Apologies for the screed, but this afforded me an opportunity to think about DMing in a more systematic way. It's not so much that DMing skills are "hard", it's that DMing requires an array of skills that is rare to find all in one person. Like anything, work is required to hone the craft. I mean, is learning to play the guitar "hard"? I can't do it, and I find it mystifying, but hundreds of thousands (millions?) have done so by putting in the effort.
