Why did the Briton's speak Welsh?

Answer 1

If you mean the ancient Britons who were native to the Roman province of Britannia, they did not speak Welsh. They spoke a Celtic language known today as Brythonic or Brittonic, and they were a people who called themselves Pritani or Britani probably from the early Bronze age onwards.

This language is the ancestor of Welsh and Cornish (Kernowek) but there are many linguistic differences.

When the Roman Empire withdrew its support for the province of Britannia and all military and political structure collapsed, the migrating Saxons, Jutes, Angles, Wends, Frisians, Franks and others who arrived on these shores gradually pushed many of the native population towards the west, into Wales and Cornwall.

Here their Brythonic language slowly evolved into Welsh and Cornish, but Welsh as a separate language did not exist before about 550 AD ("primitive welsh"), becoming Old Welsh by the 9th century AD.

So ancient Britons could not have spoken Welsh, since it did not exist at that time.

English...............................Brythonic.............................Welsh

river....................................abona.................................afon

water..................................dubro..................................dŵr

border.................................canto-.................................cant

fire.......................................taneto................................tân

journey...............................sent, hent...........................hynt

white...................................wen....................................gwyn

yew tree..............................ebor....................................efwr