What has caused the English spoken today to be different from the English spoken in earlier centuries? How has English evolved and why has it evolved?
Mainly through colloquialisms.
I'm no linguist, but a few of my teachers have been very knowledgeable about this topic and have been very eager to share. From what I understand, as people use abbreviations or slang, or misuse a term, and it slowly becomes more and more common, the older ways of saying those words become archaic and out of use. I would suggest doing some further research on this! I'm sure there are plenty of other things on the net that will help answer this question.
Here's a helpful link!(https://www.linguisticsociety.org/content/english-changing)