Because PHP 7.4 is End-Of-Life and it is no longer supported. See https://www.php.net/supported-versions

When a PHP version is no longer supported why should we support it. It exposes your installation to potential security risks nobody is going to fix.

Also we might use newer features of PHP which aren't available in older versions. We're only a small team of volunteers and supporting the main version of Elgg is already a lot of work. Making a version for older PHP versions is just not doable.

Any real reason?  Hard to believe it is ever supported, given the fact there is no money in it, it was once on store shelves, now it's a free drag download.  There is no reason to feel motivated, I wonder if the programmers and site runners are paid.

I may agree 5 would be totally off, 

The bad habit of theirs to remove good things and then place in something vulnerable is also plaguing