It is about the math. As soon as a person is looking for a quality or feature that is above-average – in other words, less common – there are fewer such people in the population. If a person seeks several qualities at a level that is above average for that particular feature, the number of people per, say, one thousand will be vanishingly small. For example, the number of people in a frum community who are six foot tall, highly attractive, and work as a professional, is small.
If you were to add additional specific requirements, it may become like searching for a needle in a haystack. Then consider that the other person has to want to be with you. That is why it is important that people focus on those qualities that are genuinely important to them and those qualities that contribute significantly to a successful marriage. If you are looking for someone who is statistically very rare, you are most likely going to find the process difficult.