Because for every old and broken thing, there were 100 good old cards I still want to use. I see Block as a threefold plan; first, it makes stuff cycle out, so that we can get some new stuff, and HAVE to get new stuff. You wouldn't want to watch the real events, and just see the same old deck trounce for 5 years. It also gets old broken stuff out, without needing a ban list as long as my arm. Second, it allows them to make money. If you are playing this game for anything more than sheer amusement, with friends who do the same, more than likely, block format will be in, and that means you MUST keep buying new stuff, even if you just watched your element of choice get flushed (Fire, much of the time), or can't play your narrow theme (I like Sharingan/Mill, for instance), because all of the pieces just rotated, and won't get a reprint right away. In this way, they help make sure you stay invested; even if the best card you've ever gotten is in your hands, you have to keep buying in, sort of like a big MMORPG. Lastly, it gives them a chance to, in between new effects/themes reuse old effects, thus limiting the need for creativity. If an effect is really good, but the one with it is rotating out, they can make a new card, and use the same text, which is the Yugioh plan (I like older Yugioh very much, but can easily line up a large number of different cards with identical effects). This lets them reuse old staples, while dooming broken sauce.
As to why I prefer Unlimited, there are specific characters and themes I like, and would rather keep using. Case in point, I am working on my Izanagi Unlimited Deck. This needs Sharingan stuff, Tree stuff, and the characters to use them, but rotation will leave us with only a few Sharingan ninjas (the regular three, plus two Madaras and a Danzo, maybe Obito), and next to nothing for their jutsus, and two Tree Ninjas with two whole Tree jutsus. I like Unlimited because it doesn't make the value of my binder of stuff plummet, just because they rotated out, and my buddy who is perpetually broke doesn't have to keep buying; he already has a Water deck that ROCKS, and I hate it every day. I accept that events aren't friendly with it, but I've never been so good as to need to worry about those events, anyway, and no theme I really like is competitive enough.