Why there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT clause?

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This limitation, which was only due to historical, code legacy reasons, has been lifted in recent versions of MySQL:

Changes in MySQL 5.6.5 (2012-04-10, Milestone 8)

Previously, at most one TIMESTAMP column per table could be
automatically initialized or updated to the current date and time.
This restriction has been lifted. Any TIMESTAMP column definition can
have any combination of DEFAULT CURRENT_TIMESTAMP and ON UPDATE
CURRENT_TIMESTAMP clauses. In addition, these clauses now can be used
with DATETIME column definitions. For more information, see Automatic
Initialization and Updating for TIMESTAMP and DATETIME.

http://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-5.html

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