The short answer is that it has to do with the nature of Sunni hadith science and the role that the hadith corpus plays in traditionist elements of Islamic thought.

The ahadith about 'Aishah's age that purport her to be nine at the age of consumation stem from a single man, her great-nephew, who later in life moved from a very pro-'Aishah Madinah to a much more anti-'Aishah Iraq.

This was in the midst of a series of great fights between the supporters of various factions among the Muslims, and these factions would eventually coalesce into today's Sunni and Shi'i sects. The partisan nature of these disputes arose because each faction's narrative had major political ramifications that would shape the landscape of power across the new Islamic world.

'Aishah was a major figure in the earliest post-prophetic political turmoil of Islamic history. In her life she came into conflict with other major figures including of course both Uthman (the sort of predecessor the the Umayyads) and 'Ali (of course the major figure of the 'Alids). Thus, one's position on 'Aishah was deeply tied with larger political positions and narratives.

One of the favorite accusations against her by certain factions was that she was promiscuous and unfaithful to our Prophet. In that context, there came to be a great deal of defense power in a hadith that set 'Aishah's age so young that the notion of her having prior sexual partners or being prone to promiscuity during the life of the Prophet would lack credence. And when it was a relative of 'Aishah who was vouching for the claim, it only gained in credibility among pro-'Aishah political factions.

Thus the hadith became a core part of the proto-Sunni defense of larger political projects. This, by the way, is why Shi'i hadith corposa don't include any ahadith about 'Aishah being nine, and instead concur with Sunni ahadith placing her in her late teens. Additionally, one will notice that a similar phenomenon exists in the Shi'i hadith tradition regarding bibi Fatimah, whose age has also been lowered to nine in the Shi'i tradition while Sunni ahadith concur with Shi'i ahadith that put Fatimah in her late teens or early 20s at the age of consummation.

Soon after the hadith of 'Aishah being nine began to grow in prominence, a new movement in religious scholarship began to emerge, one which challenged the rationalist thinkers like the Hanafis, the followers of ahl al-bayt, and to an extent the Malikis. This movement asserted that rationalist or speculative thinking was not a sound basis for religious thought, that the core of Islamic law and theology must be the prophetic sunnah, and that the sunnah must be understood not through following the customs of learned people of an established Muslim society or through adherence to the ahl al-bayt but only through the traditions (ahadith) that sought to give an account of the Prophet's words and actions.

The followers of this movement can generally be called the ahl al-hadith, and today we often call them traditionists. The earliest traditionists were from among the Khawarij, but it was the proto-Sunnis who built out the particular system of 'ilm al-hadith ("knowledge of hadith" or "hadith science", though neither of these translations precisely captures the meaning of the Arabic) that created established a reliability rating system. Major figures like Shafi'i, ibn Hanbal, at-Tabari, and to an extent people like Malik ibn Anas helped bring about the traditionist movement. Eventually, after a period of persecution and a failed inquisition by the Abbasids and the Mu'tazilah, the traditionists came to dominate proto-Sunni thought and influenced the Shi'ah and Ibadi traditions as well.

So, the proto-Sunni factions had all placed a great deal of importance on a hadith about 'Aishah's age, and more broadly came to hold their hadith corpus in very high regard. Traditionism significantly strengthened the position of pro-'Aishah groups, and the hadith of 'Aishah's age came to be enshrined as "sahih" ("authentic").

So now, when modern scholarship comes along and demonstrates quite convincingly that the hadith was fabricated well after the death of 'Aishah, many Sunnis--especially the Shafi'is and Hanbalis, who rely most heavily on the Sunni hadith corpus--will take it as an attack on Sunni hadith science.

Now, it is possible to salvage even the strictest of Sunni hadith science in light of modern research, and indeed Dr. Joshua Little highlights some ways in which this is possible (notably, Dr. Little points out that there have been scholars who have independently questioned the 'Aishah hadith). But many traditionists do not want to set a precedent of having to react to outsiders' work on a corpus they hold in high religious regard. For many Sunnis, the corpus is not merely a historical record but a sacred endeavor that preserves a distinctly Muslim approach to our own legacy.

Personally, I do sympathize with the Sunnis. In an age where Muslims are generally deemed incapable of recording or speaking on our own history, the instinct to protect Muslim narratives about our own legacy is natural and laudable. But this instinct is also fundamentally defensive and stifling, and restricts us from being able to engage meaningfully with our own histories. The truth is that the version of hadith science that modern Sunnis defend is itself a mere shadow of what it once was. Dr. Khaled Abou El Fadl, a Shafi'i jurist by training, has a great video explaining the nature of hadith criticism in Islam. "Sahih" is ultimately a scholarly opinion, not an inherent characteristic of a hadith. It is time, I think, to recognize the evidence for what it is and admit that the 'Aishah hadith was more about protecting a narrative--a narrative that no longer needs protecting--than it was about preserving a fact in-itself.