The species changed to adapt to climatic and genetic changes. As “better” more adapted species evolved, the less adapted ones died out.
A simplistic answer that hopefully you can understand.
If not, then do some actual reading.
You left out the part about the millions of transitional - between species - fossils.
Why don’t you give us some simplistic evidence of them?
Pictures (photos, not your “artists” renderings) would be nice for we simple minded folk.