Well, for one, horses no longer had as much use as what they had done in previous times.
Bigger, faster, more reliable machinery meant that thing could be done much faster, and in a very effect way, so if needed, farmers etc could increase much more crops.
Some of the equipment horses were attached to wasn't very safe. Large tools injured horses and humans, and sometimes, the tools just couldn't do the work. Sometimes in severe weather conditions, the plough could help more than what a human could withstand, whilst controlling large animals. Even Ox were used in some areas, too.
I know It looks to much a little, but Hope this helps