Ed Miliband managed to get something like 34% of the first round votes, though. Cooper/Burnham are touted as being around the low 20's, if even that at the moment, due to how well Corbyn's doing.
Also, this leadership election has been a lot more controversial and divisive than the last one, where a lot of disagreements were perhaps smaller and less significant.
Not to mention that the contest until now has largely been a farce, with the whole purge that's been going on, people who have been purged not getting their money back, and the sheer number of people who seem determined to discredit Corbyn, yet are incapable of doing so.
A lot can change, but someone like Burnham or Cooper managing to retain all the Corbyn voters would involve them being a convincing, capable leader who's able to turn things around. And there's very little evidence that it would be the case if they were to take over.