To quote a couple of posts from this Slashdot discussion (
Apple Disables Trim Support On 3rd Party SSDs In OS X - Slashdot)
by m.dillon (147925) on Sunday November 16, 2014 @02:24PM (#48397343)
TRIM has numerous problems, not the least of which being drives and/or filesystems which do not implement it properly. Because its use and effects can be seriously non-deterministic (even in a proper implementation), any bug in the drive firmware OR the filesystem in the use of TRIM can create serious corruption issues down the line when the drive actually decides to blow away some of the trimmed sectors. The TRIM command was badly conceived from the get-go.
by ericdano (113424) on Sunday November 16, 2014 @03:42PM (#48397795)
The latest episode of ATP, they heard from an Apple Engineer that Apple disables it because most makes of SSD are very inconsistent on how the TRIM command is executed. And Apple being Apple, they don't particularly want to try every SSD known to man to "support" them.
by TheRaven64 (641858) on Sunday November 16, 2014 @07:12PM (#48398905)
The reason that Apple disabled this is that a lot of SSDs have really buggy TRIM implementations. This observation wasn't unique to Apple: Microsoft and the Linux kernel defaulted to TRIM being off until quite recently. Apple could afford to turn it on for their own SSDs because they did extensive compatibility testing of those before shipping them.
Now, it doesn't really make sense, but enabling it automatically would likely burn some users, and bug reports about data loss lead to a lot more anger than bug reports about lower performance.
by Anonymous Coward on Sunday November 16, 2014 @02:18PM (#48397305)
The Linux kernel, for instance, keeps a blacklist for this issue instead — but one that (commonly) only grows when the devs get reports from somebody who already suffered data loss, and then it takes ages for the new kernel to be used widely in the wild.
In particular, I'd like to make sure you read the phrase that read
"Microsoft and the Linux kernel defaulted to TRIM being off until quite recently. " 
So please guys, less nonsense Apple bashing and more thinking about the perfectly valid technical reasons why Apple might have done what they did !