The Scot replaced England-bound Sam Allardyce at the Stadium of Light in the summer of 2016 with the club in the Premier League.
But his difficult 12-month spell ended with the Black Cats relegated from the top flight for the first time in a decade.
Moyes has praised the club's "passionate supporters" and described Sunderland as a "brilliant club".
But he says he didn't have the funds required to rebuild the team and said it was difficult to "attract a level of player to make the difference."
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Sunderland had debts of more than £125million towards the end of Ellis Short's spell as owner, with spending on new players limited.
Yet Moyes did have a healthy transfer budget in the summer of 2016 with the club forking out £13.6million on Didier Ndong and £8million to Chelsea for Papy Djilobodji, yet their time at Sunderland has proved disastrous.
Following relegation to League One, both players went AWOL this summer, with Sunderland last week informing Djilobodji they intend to terminate his contract after he finally returned but out of shape.
Ndong remains AWOL.
Reflecting on his Sunderland spell, Moyes told the Sun on Sunday: "It really is a brilliant club with passionate supporters, but they had big financial problems.
"We didn’t have the funds required to rebuild the team and it was also difficult to attract a level of player to make the difference.
"We couldn’t get it going, but lots of good managers couldn’t either.
"After I left, it sadly continued on a downward spiral."
Short's final act as owner was wiping £125.7million of debt, with Stewart Donald taking charge at the Stadium of Light this summer with the club now in League One following successive relegations.