Why is it imperial to keep PHP4 compatibility if great majority of hosting providers have already migrated to PHP5?
Because the numbers we see suggest that a large percentage of users are still using PHP4. It would not be a very smart move to move to PHP5 exclusively, alienating a number of users, ultimately gaining very little. After all, if you are using EE as such, you do not need to be concerned with what PHP version is used underneath and if you want to use PHP yourself, in your templates, plugins and such, you can certainly use PHP5.