Originally Posted by Michael J Fox is Canadian
Presumably doc tried everything he could with the 'broken' delorean before burying it (as he writes in his letter). But consider that had the time circuits still worked when he was initially sent back to the old west, he would have only made more time travels; to 1955 to get Marty and then back to 1985 before destroying the delorean. Even still it wasnt a big deal for him, he seemed happy in the old west. It was likely clara who pushed him to re-create time travel.
The only reason I thought he could repair the Delorean was because I thought maybe he could gotten working parts from the Delorean Marty brought back. But that's a big maybe.