To answer the question at the end of this: Get the cheaper option, it's just as good.
To answer the thread question: Because you are told to spend on it. Everything about weddings is jacked up to the maximum to take advantage of you. Trust me on this one. I'm a wedding DJ. The normal price I would charge for DJ services is literally jacked up about 500% or more for weddings. Why? It's called the "Wedding Tax". As your wedding DJ, there is a LOT I have to do, and you are paying for my time. If this wasn't a wedding, you'd spend a fraction of what you'd spend, and get the exact same quality. You're mostly paying for my time (and Asshole Fees, because you people are crazy about weddings). If I know you, I'll charge you normal DJ rates, because I know weddings are an expensive in endeavor. To give you an example, I normally charge about $100/hour for restaurants and most parties for DJing, and a 3 hour minimum. Of course, I'm willing to negotiate (within reason) to fit budget. For weddings, I charge about $700/hour, with a 3 hour minimum. I won't even take your phone call if you're not willing to spent at least $1000. Mostly because I want it to be worth my time. Especially with the trends going on in weddings now (which completely screw most DJs), because it makes it hard for us to power equipment. I also rely on these to pay my bills, so, that is another reason on weddings.
Giving you a better idea: