Propably because Sunday has been a day off from the work. Then it´s time to spend more time for cooking, gathering family and friends together and spend some free time together.
Wikipedia says this:
Eating a large meal following church services is common to all of Europe and other countries with a Christian heritage. On Sundays, all types of meat and dairy produce are normally allowed, unlike Fridays where many Roman Catholics and Anglicans traditionally abstain from all meat but fish. Likewise, it is traditional for Anglicans and English Catholics to fast before Sunday services, with a larger meal to break the fast afterwards.
There are (at least) two opinions on the origins of the modern Sunday roast. One holds that, during the industrial revolution, Yorkshire families left a cut of meat in the oven before going to church on a Sunday morning, which was then ready to eat by the time they arrived home at lunchtime. The second opinion holds that the Sunday roast dates back to medieval times, when the village serfs served the squire for six days a week. Then, on the Sunday, after the morning church service, serfs would assemble in a field and practise their battle techniques and were rewarded with a feast of oxen roasted on a spit.
By this it partly seems to have something about the Christian background but I think that all people have to eat something - also at the Sundays. And roast sounds tasty.