Because the day of Eid is not known to them in advance, so it is difficult for them to tell their bosses the specific day that they want to take off.
Eid prayer is fard kifaayah (a communal obligation). If sufficient people undertake to do it then the rest are absolved of sin. Some of the scholars are of the view that it is fard ‘ayn (an individual obligation) like Jumu’ah prayer. Because the Islamic centers organize the Eid prayers according to the sighting of the moon, those who cannot attend are absolved of this communal obligation. It is not permissible to delay the prayer until the second or third day of Shawwaal so that all the Muslims in London can attend, because this delay is contrary to that on which the Sahaabah and those who came after them were agreed. We do not know of any scholar who suggested this. Yes, it is permissible to delay the prayer until the second day if they did not know it was Eid until after the sun had reached its zenith.
And Allaah is the Source of strength.