Quite simply, the Constitution provides that the several Legislatures of the States shall set the rules to be followed in their respective electoral processes.
If some do and some do not, then the Constitution is not being equally followed, and the Equal Protection Clause may be invoked.
It is a call for all States (and Commonwealths) to follow the Constitution, and for their respective legislatures to establish those election rules, not for the edicts of governors, election supervisors, or others to usurp that power. Twenty Four Senators from PA have filed an Amicus Curie brief with SCOTUS on the side of Texas. In addition to Texas, at least seventeen other states have filed Amicus Curie briefs in support of Texas' suit.
If you read the claims by the states supporting the defenders,
http://www.gopbriefingroom.com/index.php/topic,422131.msg2342684.html#msg2342684you will find that one item they are claiming is that that each state decides what is the legislative process within the state, and even though the Constitution says
legislature, the state can change that to mean
a legislative process for the purposes of elections.
Some people just cannot read.