Why did the Obamas take so long to endorse Kamala Harris?

Barack and Michelle Obama have at last endorsed Kamala Harris for the Democratic nomination. It took them five days to ring her and in a publicised call congratulated her and promised their endorsement. Kamala was duly gratified. But she, like everyone , must have been thinking: "Why did you take so long?" Biden endorsed her straightaway, so did Nancy Pelosi, the legendary former Speaker of the House. The Obamas sounded like it was the simplest thing to make the call and wish her well but what was going on between Sunday when Biden stepped down and today when they picked up the phone? By all accounts they weren't happy with Biden's decision to endorse her without considering the alternative which was to hold a primary with other candidates coming forward. In other words, they didn't really rate Kamala and wanted others to get a chance to go for the nomination. But Biden's announcement put paid to that, so the Obamas had little choice but to phone and endorse Kamala or risk the damaging whispers that would have built up into a roar if they remained silent. Did they want Gretchen Whitmer, the feisty governor of Michigan to get the nomination? I suspect they did.