Yes, this is in the book Kamil al-Ziyarat:
The narration is as follows: I said to Imam Sadiq (A), “May I be sacrificed on you, I cannot afford to go to Hajj every year.” The Imam (A) replied: If you ever wanted to go to the Hajj but could not afford it, go to the Ziyarat of Imam Husain (A), for indeed it will be recorded as a Hajj for you. And if you ever wanted to go for Umrah but could not afford it, go to the Ziyarat of Imam Husain (A), for indeed it will be recorded as an Umrah for you.
As for why? Allah knows best.
However, some thoughts. First, it is not neglecting hajj, only discussing the situation where it is impossible.
Second, it is also said that worshipping at Masjid al-Kufa has a similar reward to hajj, and Masjid al-Kufa is associated with Imam Ali (A) so this general idea is not only said about Karbala (although it is more emphasized for Karbala).
Third, if someone from afar cannot perform hajj, then probably they also cannot go to Medina and visit the Prophet (S), so it is not worth mentioning here.
Fourth, there are many narrations that indicate there is something spiritually unique about Karbala and where Imam Husain (A) was laid to rest, and visiting Imam Husain (A) is emphasized above visiting the resting places of the other Imams. So, it is good to trust in that and assume there is in fact a hidden merit to visiting Karbala which is not found in other places. There are many aspects of the spiritual world that we don't see with our physical eyes and in our everyday reality. Of course, it is also very good to visit Imam Ali (A)!
The best way to answer this question for one's self is to go to all these places (hajj, Najaf, Karbala) and develop a personal view. If you have not, may Allah bless you with the opportunity!