When the Temple stood, one who offered a sacrifice was prohibited from performing melakha (productive work) that day, as it was like a holiday for him, and it is inappropriate to perform melakha while one’s sacrifice is being offered up on the altar. The same applies to Erev Pesaḥ, a day on which Israel is obligated to offer the korban Pesaḥ (Paschal sacrifice), and which is consequently like a holiday for everyone. But since the time for offering the korban Pesaḥ does not begin until midday, the prohibition against melakha also does not begin until midday. Even after the destruction of the Temple, this enactment remained in effect, and it is forbidden do perform significant melakha after midday on the 14th of Nisan. Another reason for this custom is to prevent one from becoming preoccupied with work and forgetting to destroy his ḥametz and prepare matza, wine, and other necessities for the Seder night.
In some places, people adopted the stringent practice of refraining from melakha from the morning of the 14th of Nisan, and in such places this custom was binding. In practice, however, the prevailing practice everywhere is to do melakha until midday.
Indeed, the Sages instituted a ban on significant melakha every Erev Shabbat so that people would be free to prepare for Shabbat. However, the prohibition on Erev Pesaḥ is more severe, for while the Sages taught that whoever performs melakha on Erev Shabbat will see no sign of blessing from it, those who performed melakha on Erev Pesaḥ were actually excommunicated. In practice, the halakha is that melakha is forbidden on Erev Shabbat from the time of minḥa ketana (two and a half seasonal hours before sunset), while the prohibition on Erev Pesaḥ begins at midday.
See MB 468:12 and Kaf Ha-ḥayim 32 ad loc. regarding the modern-day custom regarding Erev Pesaḥ before midday. The fact that they wonder about the custom in Jerusalem makes it clear that there is no fixed custom to prohibit work, since where work is prohibited, it is also prohibited for guests, and certainly for the residents of the community. There are no communities nowadays that prohibit work on Erev Pesaḥ before midday.
However, according to SA 468:5 in the name of Rambam, even where there is no custom to prohibit work, only three types of craftsmen are permitted to begin working before midday: tailors, barbers, and launderers, since their work is done expressly for the holiday. For everyone else, if one began a task before the morning of the 14th and the work is necessary for Pesaḥ, he may work until midday, but if he did not begin before the morning of the 14th, he may not start. According to Rema, based on the opinion of most Rishonim, including Rashi, Raavad, Rosh, Ran, and others, the aforementioned distinction only applies where the custom is to refrain from work (like in Rema’s community), but where the custom is to permit work until midday (like in our communities), everything is permissible. This is the custom of the Ashkenazic communities. Even some Sephardic communities follow Rema; see Kaf Ha-ḥayim ad loc. 32. In any event, we follow the lenient view when there is uncertainty regarding a rabbinic law.