Do other roasts not taste good if they're made in an espresso machine?
Dark roast has a heavy bitter taste. As espresso usually is served pretty strong, people expect the bitter taste. Most bars serve 70/30 or 80/20 (natural/dark roast) blends. If the roast is made of the cheaper robusta bean, the coffee shop can save money.
But, you can make an excellent espresso from 100% natural roast arabica beans (I do it all the time). People will notice the difference, even though they're not conscious what it is.
Bitter taste is not from the degree of roast. It is from the oils produced when you pass second crack.
There is a way to enjoy a dark roast espresso without the bitter taste: On your machine allow ½ a second of liquid to come out before placing the cup. Done, simple.
The oils are the first thing that comes out of the machine and for only about half a second.
First let's be sure we are on the same page in terms of light and dark.
When I talk about a light roast, I'm thinking of what is called a City roast in roaster's terminology. This is the lightest you can roast a bean to keep maximum origin flavor (you taste more of the bean's taste and less of the flavors from roasting). Any lighter, and the coffee mostly tastes grassy and sour. Sour flavors diminish as beans roast.
In my mind, a dark roast (for me) is what is called Full City in roaster talk. There are darker roasts above Full City but before the beans ignite in the roaster (French and Vienna). To me, these don't exist. I find them acrid, bitter, and lacking any of the interesting flavors I source good coffee beans to experience.
Here is more detail on degree of roast from a home roasting site / green coffee bean purveyor: http://www.sweetmarias.com/library/content/use-all-five-senses-determine-degree-roast-0
If that link is broken in the future, a quick search for degree of coffee roast should yield a good explanation.
This is all important, because one person's dark roast could be another's light. Moving to use this more standardized language described in the link, empirically, I find most City to City+ roasts still fairly sour as espresso shots. That said, sourness varies with bean age, bean origin, and growing season to growing season. We are talking about a crop here, similar to wine vintages.
So, I do not think it is fair to say all City roasts are unsuitable for espresso. Maybe you have found some beans that display beautiful fruit flavors and are not particularly sour. Or the original flavors are so interesting that the sourness is worth it, and a little sugar minimizes it. In my experience, espresso does a good job of highlighting those origin flavors even at the Full City range, which I find less true with other brewing methods. So, when I roast my coffee, I typically have a City / City+ level roast for french press and pour over and a Full City level roast for espresso. That said, I've definitely pulled good / interesting espresso shots from City+ level roasts from beans that are not overly sour.
In summary, there is no technical reason you cannot pull an espresso shot with a very light roast. Light roasted beans tend to be harder, but for a good grinder this is a non issue. Really, the question comes down to taste, which is subjective. If the sourness of a light roast is a non-issue for you, or not true of the beans at hand, and you like what you are drinking, then brew and enjoy.
For most people, dark roast is best for expresso. Mother Nature has provided us with a range of flavors to enjoy and to be sure, "bitter" is one of them. For example, it's what makes your cola taste so good. This is something I would not have appreciated as a child, but as your taste matures you come to value it. The same can be said for "hot" as in Mexican or Thai. And did someone say "burnt"? Burnt is one reason I am addicted to Starbucks Cafe Americano - strong, bitter, and a slightly burnt taste moderated with half and half. It is an elixir of the Gods. So just as for hot foods, for those who don't appreciate bitter, you have my sympathy.