Richard III might have been the last English Monarch but he was far from the last leader of a nation to lead (and die) in battle. King Gustavus Adolphus of Sweden for example was regarded as a military genius of his time, lead the Protestant forces in the 30 years war and was shot to death in the Battle of Lutzen. (Mid 1600s)

In other cases some leaders of nations were involved in the military before they became the leader. Many US presidents for example were former military men though I believe that George Washington was the only one who led forces into battle while being president (I may be wrong about this.) (Late 1700s)

Napoleon Bonaparte is another famous example of a leader of a nation still leading troops into battle. (Early 1800s)

As to why the practice became less and less common however there are multiple reasons, yes guns and more modern technology did make its presence known but a commander/general/king was not in much more danger of early muskets than they were of a crossbow or bow and arrow. One of the reasons many kings of Europe stopped participating in battles was that their position as king and succession was much more solidified which meant there was much less chance of a general performing a military coup against them as very few would view a rogue general as a legitimate ruler of a nation (not that it was impossible for such a thing to still happen). Another major reason was that it stopped being an expectation amongst many cultures for a leader to be a great warrior/military genius.

Hopefully this helped a little but the tl;dr is that around the 1800s kings/queens were simply not expected to lead armies (in Europe) and instead focus on actually running the country while the military generals took care of the fighting.