There have been numerous debates in the suggestion forums over many seasons about this issue, some involving myself.
Essentially the argument against changing it is:
The transfer market.
If players couldn't play finals after all-star break, then there would be significant periods of time during a season with which the transfer market would be flooded with players, and prices would fluctuate to a point where the transfer market would become a joke and an integral part of the game and game economy would be lost and you would then pretty much write the game off as being unplayable. Any major implications to the economy of this game would kill it and BB know it.
It is of my belief that BB think that they would lose more users (ie: me and you) to the game if they made this change and resultant transfer market implications, when compared to current save-n-splurge tactics which drive other users away.
The other argument commonly said amoungst forums around this discussion is - There is no one stopping you from using the same tactic. Everyone can do it, and can get exposed to the same risks. Everyone is on the same playing field. So its not unfair, its a legitmate game tactic that is available to everyone.
Again - not neccassarily my beliefs.
I dont totally agree with it all, just summarising some things that came out of many discussions.
It is my personal belief that even one week earlier in the season for playoff cut-off time would be enough to stem users from doing the save-n-splurge tactic, without too much implication to the game's economy. All-star break is too long and would cause the above economy clusterphuk-phenomena and yer.
Even before i had played out a full first season, i remember thinking to myself 'man, i can buy players this close to playoffs? thats a bit wrong isnt it?"...
so yer, it aint natural. But too big of a change would spell disaster.