Because Internet Explorer was included with Windows for so many years, and was integral in the process of updating the Windows OS, Microsoft had an effective monopoly on browser software and became complacent. Hackers, meanwhile, were anything but complacent. That combination led to a browser that had few if any new features added from one version to another, and few versions were released, but security patches had to be added constantly because hackers were out to get Internet Explorer, possibly because of Microsoft's monopoly.
When Firefox and Google Chrome came along, they demonstrated what a web browser could really be, and they frequently added new features and/or improved on old features. Internet Explorer started to look primitive by comparison, and after five years of Explorer 6, Microsoft came out with Explorer 7, which immediately froze up the first time I opened it. I just now opened Explorer to see what version I have ( I don't even use it anymore so I lost track). I have version 13, and when I open it, I get all these pop-ups "warning" me that a website (the one I designated as my home page) wants to use Java Runtime version 8. I click "Allow" and get a pop-up saying the website wants to use Adobe FlashPlayer. I click the "Allow" button, and I get the message two more times before it stops warning me about Adobe FlashPlayer, and then it warns me about Java Runtime again. When I finally get rid of the pop-ups, what I see is a browser that's a poor attempt to copy Google Chrome, and even then it's trying to copy Google Chrome from five years ago. I hear Microsoft is scrapping IE after all these years. Good!