The root of the difference between Sunnis and Shi'is is that a Shi'i believes that the Prophet appointed Imam 'Ali as his direct successor in all ways (political, spiritual, practical, etc), and a Sunni does not. (Even if the Sunni might consider Imam 'Ali more meritorious than the first three caliphs, or might consider Imam 'Ali as the Prophet's spiritual successor, etc.) And 'Eid al-Ghadir is essentially a celebration of the Prophet's final public declaration of Imam 'Ali as his successor.
Many Sunnis have a different understanding from Shi'is of what happened at Ghadir, both historically, in terms of the interpretation of the hadith of Ghadir, and in terms of the understanding of Qur'an 5:3.
Also, 'Eid al-Ghadir is specified as an 'Eid by Shi'i hadith. That is, Shi'is do not independently decide "let's celebrate 'Eid al-Ghadir" or "let's call the Day of Ghadir an 'Eid", but rather it is specified in hadith as an important 'Eid, and in fact the most important 'Eid.
However, Sunni hadith do not specify the day of Ghadir as an 'Eid. There is a narration in in Tarikh Baghdad which speaks of the "Day of Ghadir" and recommends fasting on that day. However, Tarikh Baghdad is not a major source of Sunni theology, and, in any case, it doesn't specifically say "'Eid al-Ghadir" (vol. 8, p. 284).
So, one could also say that both Shi'is and Sunnis are following what is in their hadith sources.
Of course there are exceptions especially in areas where there are blurrier lines between Sunni and Shi'i practice or a mixed heritage of Sunnis and Sh'ism.
The announcement of Ghadeer took place on 18th Thil Hijjah in the year 10 after Hijra. The Prophet Muhammad (SAWA) passed away on 28 Safdar year 11 after Hijra which is just 70 days after Ghadeer.
The Prophet (SAWA) did celebrate the announcement of Ghadeer in different ways like :
1. He asked every Muslim who was present ( men and women) to give allegiance to Ali (AS) as The master after the Prophet and as Ameerul Mo’mineen.
2. He asked all who were present to convey the message of Ghadeer to those who were not present.
3. The Prophet informed the Ummah that Ghadeer is the greatest Eid in Islam.
4. He celebrated Ghadeer by informing the Muslim Ummah that Islam was never complete but after the announcement of Ghadeer.( Chapter 5, Verse 3).
5. He celebrated Ghadeer by informing Muslim Ummah that fasting the day of Ghadeer has great reward. Even in Sunni books you find this Hadeeth of fasting the 18th of Thil Hijjah ( day of Ghadeer) narrated by Abu Hurayra from the Prophet (Al-Bidayah wal Nihayah, vol 7, Hadeeth Ghadeer Khum).
These are few examples from the Prophetic acts celebrating the greatest announcement of Ghadeer.
The event of Mubahala took place on 24th Dhu'l hijja 9 AH and the event of Ghadir Khumm took place almost a year later on 18th Dhu'l hijja 10 AH.
The Prophet (s) passed away within a few months of Ghadir Khumm. At the first anniversary of that event Abu Bakr was in power and unlikely to celebrate an event that would undermine his own rule.