Because NATO (for most key decisions) works by full consensus, adding new members (a process which involved Turkey and Hungary using their leverage for concessions, especially Turkey) does not substantially reduce the leverage of existing members (this is also why Russia's bid to move to the front of the line and be admitted ahead of other Easter European applicants, without a readiness process--the real root, not the fact of expansion into Eastern Europe, of Putin's resentment against NATO--was rejected.)