Aside from having to switch his stance because Biden dropped out, higher profile politicians have to plan out when they announce their endorsements. The ones that progressive and liberal voters really care about are those of Obama and Bernie. You let the candidate build up some momentum to see how they do, and she did pretty well. They also want to make sure they can actually successfully accomplish making her the new nominee, seeing as how this was a weird situation. Then, you start doling out the endorsements at opportune times. You want to spread them out a little bit, but still leave them close enough to give the public that overall impression when they’re reading the news that the candidate is still building and gaining more momentum. As you can see, it works. Excitement about her replacing Biden is a wake they absolutely should ride as long as they can. Go too fast or too slow and you lose the boost.