Quote Originally Posted by Max_Killjoy View Post
Just be honest and tell them that you'd like to try something different.
Out of the systems you're interested in, pick one that's relatively simple or easy to learn, and a campaign idea you can wrap up in a few sessions if it doesn't go well.
If they're really dedicated to D&D, tell them that playing one or more other systems will help them understand what D&D is doing and not doing.
This. If they really don't want to try something else, you're either not going to sway them, or they're going to be very predisposed to finding reasons not to enjoy it. If they're willing to at least try, don't make a huge commitment. Let them decide how far they want to go. Tell them what YOU like about the system you're interested in running and why D&D doesn't do that thing.