The easiest answer is: it was unnecessary.
Joseph was a young man, given to the brashness and bluster of youth. He sometimes got caught up in himself. That's easy for us to see, some 200 years later and flush with historical documents. Therefore, whenever I read excerpts from TPJS, I hold my judgement (and sometimes my breath) until I can get a better read on the situation.
The greek word used to convey what Jesus said in Luke 23:44 is Παραδείσῳ (transliterated: Paradeisō). It's used only three times in the ancient texts. Here, 2 Corinthians 12:4, and Revelation 2:7. Paul's use of the word is in context with the "third heaven", which we call the Celestial Kingdom. Obviously, Jesus wasn't pre-judging the malefactor or making his calling and election sure. John's use of the word is a bit more vague, as it is the location of "the tree of life, which is in the midst of the paradise of God." But, from an LDS perspective, the Tree of Life was only physical in the Garden of Eden (...paradise...) and is figurative for all other references, relating to immortality (the resurrection), not exaltation (eternal life).
Luke's use of the word, if he actually used the word (the oldest Greek manuscript is not believed to be the original), is the most vague of all.
Gratefully, we have those 200 years of experience to work with. Remember, Jesus was hanging on a cross with nails in his body. I'm sure it HURT. Even the thieves, tied rather than nailed to their crosses, were in pain. The entire conversation, which seems polite and calm in the Bible, was much more likely to be spoken through that haze of pain, gritted teeth, and spoken in bursts as their bodies gasped for oxygen to try and handle the situation. No wonder Jesus uttered a sentence rather than a sermon!
So, when we accept the reality of Jesus' situation, the use of the linguistic construct elsewhere in scripture, the reality of Joseph's personality, the fact that he often received knowledge "line upon line", and the basic skepticism with which we must take all writtings not Scripture or General Conference addresses... we have the following:
Alma 40:11-14 teaches:
Concerning the "Wicked": they have no part nor portion of the Spirit of the Lord; for behold, they chose evil works rather than
good; therefore the spirit of the devil did enter into them, and take
possession of their house—and these shall be cast out into outer
Everybody else is in Spirit Paradise.
Remember that imperfection I just mentioned? We believe in the Bible insofar as it is translated correctly. We believe that about all books. Joseph Smith taught that the Book of Mormon was the "most correct of any book on earth." But even it is not perfect. Alma 40:14 teaches that people stay in spirit paradise and spirit prison (together, "the spirit world") until their resurrection: which means he was being a bit vigorous when he said they would be cast into Outer Darkness. That's overclassifying it a bit as we have scriptures in the Bible and the D&C that teach that people are taught in the Spirit World, the purpose of which is fair judgement. Indeed, Moses described the entirety of the Spirit Wold as "prison" (a literary allusion to all spirits being held back for a purpose. Moses 7:57). But, a more detailed analysis of how mortal man isn't perfect and how that finds its way into scripture is an answer for another question. Taking what Alma taught for its intent:
A) Spirit Paradise is for people who accepted Jesus during mortality, whether they were perfect in all their ways or not.
B) Spirit Prison is for people who did not accept Jesus in their mortality, whether they were imperfect in all their ways or not.
NOTE #1: It is my personal opinion that judgement will be fair. This is why even when I was a Branch President, I was (and still am) very slow to "judge" people in this life. Who is to say that they have wholly rejected... or accepted... Jesus? Nevertheless, it is useful to understand the "limits", "boundaries", and "definitions" surrounding judgement so that we can strive to do better. Thus, I'll leave final judgement to Him and I'll stick to helping people repent.
But the other answering rebuked him, saying, Dost not thou fear God,
seeing thou art in the same condemnation? And we indeed justly; for
we receive the due reward of our deeds: but this man hath done nothing
amiss. And he said unto Jesus, Lord, remember me when thou comest
into thy kingdom.
IMHO (I love that abbreviation, "in my humble opinion...", using the phrase is an oxymoron...) re-translating Jesus' statement from "Paradise" to "Spirit World" makes His statement to the malefactor meaningless as that will happen to everyone regardless of what they do on this earth. Such an answer could even be deemed cold, merely a means of getting the guy to shut up. Jesus has never been without compassion throughout His life.
On the other hand, the malefactor who is speaking (a) recognized Jesus for who and what He was and (b) honestly recognized his own sins. That is the basis for repentance. Not the end, but the beginning of sincere repentance. This alone qualified this gentleman for Spirit Paradise and, therefore, Luke 23:44 requires no modification at all.
NOTE #2: Yes, this sounds a lot like death-bed repentance, but remember (a) Ezekiel 18 permits sincere death-bed repentance and (b) there is time in the Spirit World for the Malefactor to learn about and fully accept Jesus and His gospel. What we must remember is that Spirit Paradise is NOT simply for people going to the Celestial Kingdom. We have no doctrine as to where the separator is, but a rough, imprecise, and possibly wrong "rule of thumb" is the line between the Telestial and Terrestrial kingdoms.