Suppose I gave you a promissory note for $8, payable in December of this year. How much would you be willing to sell that for? Well, it depends on whether you think I will actually pay in December or will skip town. If you really think I will pay up, then you would probably demand a price close to $8, taking into account the advantage of having the money now rather than in several months. But if you're worried I won't be good for it, you might take a serious discount.
Based on PacBio's stock price, Wall Street is very pessimistic on the merger happening. As in my example, Illumina is offering $8 cash for each share of PacBio. Normally that would mean PacBio would trade at a very small discount to $8. But instead, it has been trading as low as $5.35 (July 30th), though it has recovered to $5.68. That's a nearly 30% discount to the offer price! I've been told by someone in the investment community that such a steep discount is essentially unheard of.
Of course, in my example you're left with nothing if my deal fails, whereas a failure of the acquisition leaves PacBio as an independent company with around a year in cash, given the breakup fee. That's much less than a year to either get financing or another deal set, as a company's negotiating position is poor if everyone knows they are running on fumes. Shopping around again largely depends on getting new interest due to the launch of Sequel II (which has been robust, with over 40 systems placed). Previously only Illumina and BGI showed interest; given the current political situation (as in the U.S.-China trade war) there'd be a whole different type of regulatory risk in going that direction.
That the investment community is so pessimistic that the deal will fail isn't guaranteed to come true. The CMA could well approve the acquisition subject to certain conditions; it would not be the first time smart money was caught stupid. But it is sobering that there is such widespread belief that a share of PacBio will not be worth $8 in the near future, despite a signed agreement to make that so.