Why is it easier to change the constitution than it is to pass a law

For the United States, it is not.

Article five of the United States Constitution.
"The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures ofthree fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate."

This process has happened 26 times and once unconstitutionally (the 16th amendment) for a total of 27 amendments. 

To pass a law, there must be a 51% vote in both houses of Congress, and an approval by the president. Or Congress may override the veto by a two-thirds vote. This has been done well over 40,000 times in 2011 alone.