I understand the same procedure was used at Sutton Hoo - leaving part of the site untouched for future archologists:
“A substantial part of the gravefield was left unexcavated for the benefit of future investigators and as yet unknown scientific methods.”
It's the usual practice. Keeps a tight focus, keeps the budget under control, and as the man sez, someday there will be better technology, analogous to today vs pre-RC dating, pre-xrays, pre-ground penetrating radar, etc. The only time a site gets completely cleared is during rescue digs on construction sites. That's done in places with crowded strata, like Rome, Israel, Greece, Turkey...