And why did they fuck around with block designs? Because Maro could not come up with enough cards to make a compelling 3-set block anymore. Time Spiral (maybe Lorwyn if you want to be generous, Ravnica 1 if you want to be strict) was the last time until Return to Ravnica that Wizards did not add some catastrophic event in the 3rd set of the block to justify making all new themes/mechanics, Return to Ravnica coasted on the original's popularity, then we only had one more block (Theros) before the two-block structure was implemented.
They also need to fire whatever idiot decided to gut quality control and decide broken cards are acceptable - if drafting amounts to hoping I get a chase rare and my constructed deck could get banned for WotC's mistakes, I'm not going to spend a dime.
Edit: I just realized two things. First, Maro became head designer in late 2003 or after Ravnica was already in development. This means Time Spiral was the only block where he was in charge that tried to follow the old 3-set structure until Return to Ravnica. The second thing I didn't realize was that Gatecrash was a large set which means Theros was the first time in six years and only the second set he created that didn't deviate from the 3-set block design in some way. Put them together and either he never had the creativity or he was eager to break the block design WotC had used for over a decade the second he took over.
I don't especially love MaRo, but I'll defend him on this one:
Pre-MaRo:
Mirage block: the first block.
Weatherlight has a (quiet) graveyard theme that isn't supported in the first two sets.
Tempest block:
Exodus is continuous in theming and mechanics with the first two sets.
Urza's block:
Destiny is more-or-less continuous with the first two sets.
Masques block: Every set is on a different world.
Prophecy has a no-lands-untapped matters theme that is at best vaguely supported in the rest of the block.
Invasion block:
Apocalypse is all enemy-colored whereas the first two sets were allied. You can argue this is an extension of what they were doing.
Odyssey block:
Torment was color-imbalanced to black, so
Judgement was color-imbalanced to white and green. Both sets were less all-in on the graveyard theme of the block than
Odyssey itself was.
Onslaught block:
Legions is an all-creature gimmick set.
Scourge has a high-cmc theme supported nowhere else in the block, and the "play lots of spells" play pattern of Storm is also pretty unsupported (even in the same set!).
Mirrodin block:
Fifth Dawn is all about playing five colors. Aaron Forsythe came out and admitted years ago they did this because of power level concerns from the rest of the block.
Kamigawa block:
Saviors has a hand-size-matters theme supported nowhere else in the block.
Post-MaRo:
Time Spiral block: aside from being themed around time and having a mishmash of MTG history in theme, every set is fairly distinct.
Alara block:
Alara Reborn is about as continuous with the rest of its block as
Legions is with its block: they're both gimmick sets riffing on their block's theme.
Scars block: this block's progression is driven by its story. The mechanics in
New Phyrexia, while different, play into the block's theme and the faction's theme.
Theros block: Theros block was a mess, but
Journey is about as continuous with the rest of the block as
Fifth Dawn was with its block. Arguably,
Journey should have been what the whole block was, but I won't soapbox about Theros's sins here.
So I think you could argue that the block system had been messed with long before MaRo was in charge, and the third set was usually the red-headed stepchild of the block system.
