Why doesn't the whole Muslim world start Ramadhaan and celebrate 'Eeds on same days?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The reason why all Muslims do not fast or break the fast on the same day is due to the difference of opinion among the scholars  about considering the different times of the appearance of the crescent moon from one country to another.  They also differed about whether or not the lunar month could be determined astronomically.

In fact, the Muslim scholars differed about the sighting of the crescent, i.e. in case it is seen in one Muslim country, are all the Muslims obliged to fast or break the fast by this sighting, or is it that each one of those countries must have its own sighting of the crescent?

However, the majority of scholars of the Hanafi, Maaliki and Hanbali schools are of the opinion that everybody has to adhere to the crescent sighting if it is confirmed. Ibn 'Umar narrated that the Prophet said: "If you see the crescent, you have to fast, (i.e. start the month of Ramadan) and if you see it (again) you have to break the fast, but if the weather is cloudy (and are unable to see the crescent) then you have to estimate it." [Al-Bukhari and Muslim]

In another narration that explains the above mentioned one, he said: "If you are unable to see the moon because of clouds, then count 30 days." [Muslim]

On the other hand, Imaam Ash-Shaafi'ee and some other Muslim scholars said that there is a difference in the sighting of the crescent from one country to another. This is the opinion of Ibn 'Abbaas who said: "This was the order of the Prophet ." [Muslim]

Among the reasons the Muslims differ regarding the beginning of Ramadan and its end (fasting and breaking the fast) is that some Muslim scholars determine the month by astronomical calculations supporting their opinion with the saying of the Prophet : "You should estimate it."  They understand it as calculating the times of the rising and setting of the crescent.

Nevertheless, some Muslim scholars are of the opinion that the difference of the sighting of the moon from one country to another has its own consideration and that the month of Ramadan is not determined by astronomical calculations.

Allaah knows best.