I have discussed this thoroughly before. One of the reasons is that a Constitutional Convention can get out of control, just as the convention to amend the Articles of Confederation did in 1787. The result could be an entirely new Constitution, like that proposed by Rexford P. Tugwell in 1974, called the "Constitution of the Newstates of America." * In it, you will see that the Federal Government would be given the power to abolish States, reducing them to administrative units with less freedom of action than our counties have today.
* The original link to this document was through the site of an anti-Semitic hate group. A friend found for me a neutral link to present the same document.