One reason is that the company would continue to burn cash on operating costs. Then there are more trials to pay for (including the ongoing ones ), and more expenses to expand the label for more indications. TLON was far too undercapitalized to get the pipeline to market and any more money WP would give them would just get eaten up in operating costs. If the label is expanded $400 million could happen. The kink is that is several years away.
I'm not saying this is fair or a good deal, but it was the best deal they could get. You have zero leverage when you have no cash. Some analyst on the CC asked how much cash SPPI was getting from TLON. The reply was they had none and the company is gone effective tomorrow. So yes it was a fire sale. SPPI was smart to wait them out until the end of June, knowing they had no cash. Not saying it is good for shareholders, but better than BK.