A long time ago, Ireland was effected by an incredible famine which riddled the country with strife.
People were starving and when people are not getting the bare essential of food, it is common for times to turn desperate. Worry turns into concern which then easily turns into panic. However, during such a time, it is likely that those who can will search for a solution that is big enough to help the most people.

I’m sure some people resigned themselves to theft or possibly lines of work that were far from ideal or even legal. But, somewhere along the way, enough people figured out that immigrating to the United States would be the best option for them. Why was that?

Well, America was relatively new. Most people couldn’t speak of first-hand knowledge of the country and what it was like to live there. However, many people had probably witnessed loved ones and friends decide to go there. And I dare to say that most of them never returned.
First off, passage to the U.S. was far from cheap. A person would either have to work hard and save for it or be fortunate enough to receive the money from a relative in order to go. More often than not, people did not travel alone. They traveled with they entire families which required even more money.
The reasons to leave were endless. The food supply in Ireland was depleted. This set in to starvation which quickly turned into disease. People were fearful for their survival especially if they saw others around them succumb to the devastation.

America offered opportunity. It offered jobs, food, shelter and what most assumed was a lesser chance of catching a lethal disease. For those who decided to make the journey, they were typically in for a month-long voyage across the Atlantic. Some unfortunately died in transit, but for the lucky who made it overseas, they were usually overcome with relief. Was that relief so palpable that it may have blurred the reality that they now faced?
It has been speculated that many Irish immigrants over-exaggerated the comfort that they first found when arriving in the United States. Perhaps they were just unburdened enough to realize that they were luckier than those they had left back home. I also imagine that after all of the effort and cost that it took to arrive, that it seemed face-saving to admit that the decision was a good one.
What is important to remember is that during this time when the American letter became the standard for communication, choices were not based on two luxurious options. They were based on a dire need and out of despair. The letters that became the symbol for this period reflect that.