If I have take a guess it's probably because it offers difference offerings and choice. Some would prefer the cPanel and WHM interface for management while others would prefer Plesk. Plus, as for as I can tell, cPanel only offers Apache as the web server (although you could get NGINX installed) whereas Plesk supports NGINX and Apache. cPanel only supports Linux where Plesk can be installed on Linux or Windows, stuff like that. But overall it comes down to what the users wants. That and because of the pricing difference. Plesk will always be the cheaper version by about 10 bucks (give or take) so it gives more options. If they've merged or kill off either Plesk or cPanel there will probably be a lot of riots and people moving to other platforms. This could also lead to even more price increases (not like price increases isn't already been happening but it would more likely increased 10 fold).