Why was Eliyahu chosen to be one of the very few who merited to go to Gan Eden alive?
There were prophets greater than him (Moshe, for example).
There were people that kept the Torah better than him (he isn't listed as one of the four people who never sinned)
Perhaps he did not really enter heaven alive. According to Radak on the last pasuk of Malachi (Hinei Anochi Sholeach Lachem et Eliyahu HaNavi):
הנה אנכי שולח לכם -אע"פ שאני מזהירכם על תורת משה בכל דור ודור, אעפ"כ לטובתכם אשלח לכם את אליהו הנביא והטעם שישיב נשמתו שעלתה לשמים אל גוף נברא כגופו הראשון, כי גופו הראשון שב אל הארץ בעלותו כל יסוד אל יסודו ואחר שיחיה אותו בגוף ישלחנו לישראל לפני יום המשפט והוא יום השם הגדול והנורא והוא יזהיר האבות והבנים יחדיו לשוב בכל לב אל השם והשבים ינצלו מיום המשפט
"Although I warn you concerning the Torah of Moshe in every generation, even so, for your benefit, I will send you Eliyahu HaNavi. And the meaning is that his soul, which ascended to heaven, will return to a created body just like his original body. For his original body returned to the ground when he ascended, each element to its base. And after He resurrects him in a body, He will send him to Israel before the day of judgement. And this is 'the great and awesome day of the Lord'. And he will warn father and son together to return full-heartedly to Hashem, and those who repent will be saved from the day of judgement."
If we say Eliyahu was Pinchas, then this could be part of Pinchas' reward.
According to the Ramchal, this special status isn't for just keeping the Torah. It is a result of purifying the physicality of the body. Also, Moshe did achieve this status in some form, although I don't know if it is lower or higher than what Eliyahu achieved.
Daas Tevunos siman 70
כי אין סוף הכונה בביאת הנשמה בגוף, לשתחיה אותו בחיי ההבל האלה, אבל עיקר בואה בו הוא, לזכך אותו זיכוך ממש, להעלותו משפל מדרגתו החמרית וחשכה אל המדרגה העליונה, להיות כמלאכי השרת. ודבר זה מצאנו כבר במשה רבנו ע"ה, שזכה וזיכך את חמריותו, עד ששב למדרגת מלאך ממש, וכבר ראו כל ישראל כי קרן עור פני משה. וחנוך ואליהו נתעלו לשמים בגופם ממש, אחר אשר זככו את חמריותם זיכוך גדול
The point of the soul entering the body is not to live this life of vanity, but rather the primary point of its coming into it is to literally purify it, to bring it up from its low level of physicality and darkness to the uppermost level, to be like the angels. And we find this by Moshe Rabbeinu, who merited and purified his physicality, to the point that he returned to the actual level of an angel, and all of Israel saw the shine of his face. And Enoch and Eliyahu went up to the Heavens in their actual bodies, since they purified their physicality greatly. (Translation mine)
I don't know how this "purification" is done, but let's say it is accomplished through refraining from engaging in physicality. It is possible that someone could have "sinned" more but in the bigger picture have accomplished this "purification" to a higher degree by greater refraining from engaging in physicality. It is also possible that they did teshuva for any sins they did, and on top of that achieved this "purification" to a higher degree.
