There isn't a "one single reason", but the overwhelming scriptures in the Bible give one reason. Sin - bad fruit, or no fruit at all, unrepentance.
John 5:28; "Do not marvel at this; for an hour is coming, in which all who are in the tombs will hear His voice,
John 5:29; and will come forth; those who did the good deeds to a resurrection of life, those who committed the evil deeds to a resurrection of judgment.
Matt 3:10; "The axe is already laid at the root of the trees; therefore every tree that does not bear good fruit is cut down and thrown into the fire.
Matt 7:19; "Every tree that does not bear good fruit is cut down and thrown into the fire.
Matt 13:41; "The Son of Man will send forth His angels, and they will gather out of His kingdom all stumbling blocks, and those who commit lawlessness,
Matt 13:42; and will throw them into the furnace of fire; in that place there will be weeping and gnashing of teeth.
Matt 18:8; "If your hand or your foot causes you to stumble, cut it off and throw it from you; it is better for you to enter life crippled or lame, than to have two hands or two feet and be cast into the eternal fire.
Matt 7:23; "And then I will declare to them, 'I never knew you; DEPART FROM ME, YOU WHO PRACTICE LAWLESSNESS.'
Luke 13:3; "I tell you, no, but unless you repent, you will all likewise perish.
Luke 13:5; "I tell you, no, but unless you repent, you will all likewise perish."
Matt 25:41; "Then He will also say to those on His left, 'Depart from Me, accursed ones, into the eternal fire which has been prepared for the devil and his angels;
Rev 20:12; And I saw the dead, the great and the small, standing before the throne, and books were opened; and another book was opened, which is the book of life; and the dead were judged from the things which were written in the books, according to their deeds.
Rev 20:13; And the sea gave up the dead which were in it, and death and Hades gave up the dead which were in them; and they were judged, every one of them according to their deeds.
1Cor 6:9; Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals,
1Cor 6:10; nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God.
Gal 5:19; Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality,
Gal 5:20; idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions,
Gal 5:21; envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God.
This is the best I can do on short notice, and although I believe faith in Jesus is required for salvation, none of the verses above say we will be judged by our faith or what we believe in. They all say we will judged by what we do.
What we practice, and how we live.
Rom 6:23; For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord.
1Jn 3:6; No one who abides in Him sins; no one who sins has seen Him or knows Him.
1Jn 3:7; Little children, make sure no one deceives you; the one who practices righteousness is righteous, just as He is righteous;
1Jn 3:8; the one who practices sin is of the devil; for the devil has sinned from the beginning. The Son of God appeared for this purpose, to destroy the works of the devil.
1Jn 3:9; No one who is born of God practices sin, because His seed abides in him; and he cannot sin, because he is born of God.
1Jn 3:10; By this the children of God and the children of the devil are obvious: anyone who does not practice righteousness is not of God, nor the one who does not love his brother.
Rom 1:32; and although they know the ordinance of God, that those who practice such things are worthy of death, they not only do the same, but also give hearty approval to those who practice them.
Matt 19:16; And someone came to Him and said, "Teacher, what good thing shall I do that I may obtain eternal life?"
Matt 19:17; And He said to him, "Why are you asking Me about what is good? There is only One who is good; but if you wish to enter into life, keep the commandments."
1Jn 2:3; By this we know that we have come to know Him, if we keep His commandments.
1Jn 2:4; The one who says, "I have come to know Him," and does not keep His commandments, is a liar, and the truth is not in him;
1Jn 2:5; but whoever keeps His word, in him the love of God has truly been perfected. By this we know that we are in Him:
1Jn 2:6; the one who says he abides in Him ought himself to walk in the same manner as He walked.
1Jn 3:24; The one who keeps His commandments abides in Christ, and Christ abides in him. We know by this that He abides in us, by the Spirit whom He has given us.
Rom 6:16; Do you not know that when you present yourselves to someone as slaves for obedience, you are slaves of the one whom you obey, either of sin resulting in death, or of obedience resulting in righteousness?
Similarly, why do people not go to heaven?
Heb 4:6; Therefore, since it remains for some to enter it, and those who formerly had good news preached to them failed to enter because of disobedience,
Heb 4:11; Therefore let us be diligent to enter that rest, so that no one will fall, through following the same example of disobedience.
