@rereboy:
Because there is a need to do it, if they want to hope to continue the manga and finish it. If they want to risk not continuing the manga and not finishing, it's their call, but I would say that any fan who prefers that to having another good artist for the art is kind of crazy.
And they are not taking this risk, and so as other publisher as well. If the series put hiatus due to whatever reason, then they'll just leave it like that until the author able to start again. Plus, it isn't thing like Togashi is really, really old or his health is in critical situation. The publisher doesn't really think greatly series to complete by force, they want to do it by naturally. If the publishers were obsessed with completing the series no matter what, then they should had done that like long time ago. But, I don't see why they have to do like that.
@rereboy:
And it's known that it's mainly due to his health problems.
That's for this time only. For the previous time, there is no absolute explanation about his hiatus; if it was his health suffering to serialize the serialization for past 9 years, then they should already announced that (it isn't something to keep secret about). So, the health isn't the main issue for this.
Plus, if the health is main issue, then the guy needs to relax and not thinking about his work, not forcing him to do the labor.