Why does the Memorized Payee list keep getting overwritten?

To stop Quicken from doing that you can click on the Memorized Payee and then check the box for it on the right in the "Lock" column.

If you wish to make some changes to that payee listing before you lock it, click on the "Edit" button on the far right, make the adjustments you want to make to the various transaction fields, check the box for "Lock and leave unchanged when it is edited in a register" and then click the "OK" button.

This should stop Quicken from changing your Memorized Payee data or adding new Memorized Payees for Payees with the exact same name. 

But it will not stop Quicken from adding new Memorized Payees if the Payee Name is even slightly different (as is often the case in downloaded transactions).  To stop this from happening I suggest you set up Renaming Rules for Payees so that variations in the downloaded Payee Names are always entered into your registers with the same Payee Name.  For instance, if you make charges for gas at a variety of Exxon gas stations, each one might have "Exxon" in the payee name but also includes the store numbers.  You can set up a Renaming Rule so that every transaction that includes "Exxon" in the downloaded Payee Name will be entered as only "Exxon" in the register...all the store numbers will be removed.  That will help to keep your Memorized Payee list more orderly and controlled.