Why both T568A and T568B standards?

Solution 1:

The answer is that both schemes are around (and will probably remain around for the foreseeable future of CAT5 cable) for hysterical raisins. (The Wikipedia article alludes to the history here)

Basically the T568B standard is dead & deprecated at this point, but my advice is that in the absence of any specific guidance (i.e. when you're not using a punchdown block that has color/stripe codes marked on it) use the standard that is the most prevalent in your region (or company).

When using marked equipment, follow the markings.
When making a new deployment, use T568A.

Solution 2:

The reason had to do with analog voice circuits, and in particular having two separate voice circuits come to your residence.

The most prevalent in the commercial installed base in the United States is 568B (B for Bell ;).

see Registered Jack aka RJ-xx for secondary tip and ring.

New cabling should not use CAT5 / 5E. CAT6 or higher should be the choice.