Apparently, you only checked the verses in which major sins are listed. So, how come does the refusal of an obligation ordered by God become a minor sin? If refusing a clearly defined order from God is not rebellion, then what is rebellion? Rebellion means to not follow an order, to disobey.
Just as people who do the things that God forbids are committing a sin, those who do not perform God’s commands, especially about worships, are also committing a major sin. Praying, fasting, pilgrimage… If you read the rulings about these worships as a whole, you will see that those who skip these obligations without a valid excuse are in rebellion to God. Besides, in the last verse which has detailed information about fasting, God the almighty commands: “These are the bounds set by God; do not approach them! ”(Al-Baqarah/The Cow 2:187). Those who do not fast in Ramadan without an excuse do not only approach these limits, but also overstep them. God the almighty commands as follows about keeping the limits:
تِلْكَ حُدُودُ اللَّهِ وَمَنْ يُطِعِ اللَّهَ وَرَسُولَهُ يُدْخِلْهُ جَنَّاتٍ تَجْرِي مِنْ تَحْتِهَا الْأَنْهَارُ خَالِدِينَ فِيهَا وَذَلِكَ الْفَوْزُ الْعَظِيمُ. وَمَنْ يَعْصِ اللَّهَ وَرَسُولَهُ وَيَتَعَدَّ حُدُودَهُ يُدْخِلْهُ نَارًا خَالِدًا فِيهَا وَلَهُ عَذَابٌ مُهِينٌ
“These are the bounds set by God. Whoever voluntarily obeys God and His Messenger, God will cause him to enter the Gardens in which rivers flow. Therein, they will abide as immortal. That is the great salvation. Whoever rebels God and His Messenger and oversteps the bounds set by Him, God will cause him to enter the Fire, and therein he will abide as immortal. What they deserve is a humiliating torment.” (An-Nisa/The Women 4:13-14).