The first reason is beginning songwriters often don't know how long a song should be or how many parts it should have. As a result, they tend to overthink their songwriting and write too much. This can lead to songs sounding clumsy and amateurish. To avoid this, beginning songwriters should start small and work their way up. Your first song doesn't have to sound like "Stairway to Heaven." Stick to a verse/chorus/verse format, and write only four lines per verse. Once you gain some experience, you can proceed from there.
Second, people who are not musicians may have a difficult time relating to a song that's overly complicated. This is less of an issue if your audience mainly consists of other musicians, but non-musicians will not be able to hear the song from the same perspective. As a result, you risk limiting your audience. Again, there's nothing wrong with writing more complicated song structures if that's what your creativity leans toward. Still, this is something you may want to consider. Make sure your songs have sufficient elements of melody and memorability so they can be enjoyed by everyone.
As musicians, we have a tendency to think that more is better. However, when you think of songs that have stood the test of time, many of them are very simplistic in nature. The Beatles, who are considered by many to be some of the greatest songwriters of all time, were known for writing simple songs. The same can be said for many bands from the classic rock era. Although their songs may have contained advanced musicianship, many of the songs themselves were simple in nature.
So how does this relate to your own songwriting? For one, the song should contain a certain "sing along" quality. Although this obviously applies to the vocal parts of a song, it can describe other parts as well. The song also contains a few basic parts that naturally flow into one another. While it doesn't necessarily have to be verse/chorus/verse, there should be certain parts of the song that repeat themselves more than once. This helps reinforce the song in the listener's memory.
Here are a few common mistakes that you want to avoid1. Writing too many parts to a song. A song with too many parts can be difficult to relate to and usually isn't particularly memorable. This can be an option if you want to write more progressive or experimental music, but the parts should still fit together naturally. Sometimes, musicians make the mistake of adding more parts to a song for the sake of sounding more progressive. If done badly, this can be like trying to put together pieces of a puzzle that don't fit. Start small if you are a beginning songwriter, then expand later if that's where your inspiration leads you.
2. Writing verses that are too long. This is a problem I see all the time with beginners. There's no definite rule for how long a verse should be. Sometimes they should be four lines, some should be eight. Sometimes, two is enough. It all depends on what's best for the song. However, beginning songwriters often try to write more than is necessary. This can lead to writer's block, and the end results can sound unnatural. I suggest sticking to four lines per verse if you're just starting. Once you have a better feel for what works and what doesn't, then you can use more personal discretion.
3. Sacrificing melody in the name of virtuosity. There's nothing wrong with playing fast, but you don't want your listeners to think you're just showing off. You want to make sure every part of your song is memorable and has feeling. A good example of this would be the solo in "Crazy Train." While it's technically advanced, it also retains a strong sense of melody. It has that certain "sing along" quality.
Of course, all of this is more of a general rule of thumb than an unbreakable rule. A song doesn't have to be simplistic in order to be good. It's important to keep in mind the context of your genre and the emotion you're trying to express. Most importantly, you want to stay true to your inspiration.
That being said, we often forget that some of the best songs in the rock genre are easy to play and easy to sing along with. When writing your songs, try your best not to overthink what you're doing. At the very least, I encourage beginners to keep it simple in the beginning and then build from there if that's what they choose to do.