Jolla is what is known as a "startup". As such, they've received what is essentially a loan from some investors in order to create a product that will, at some time in the future, hopefully be sold in order to derive an income. Therefore, as with most startups, Jolla is essentially deep in the hole financially until they manage to sell their product.

The Indiegogo project, in terms of money, was really tiny compared to Jolla's budget. It's true, they could have immediately spent the money on tablets way back at the beginning of the year, but they (1) still needed to port Sailfish to the tablet hardware, and (2) also decided to modify the original tablet design. Both these tasks take time and money, so it wasn't until just a few months ago that they were ready to start ordering the devices. I have to assume that by that point they were already under financial pressure, and no longer had enough to order the full set of tablets (which is probably the real reason they were going so slowly with these rounds of "waves").

So yeah, everything is kind of tied together here; the Indiegogo money can't have been enough to cover all the tablet hardware and software engineering done earlier in the year, and still pay for the parts and manufacturing now. So one way or another, the rest of the bill had to be paid by their financial backers, and they just don't have the money any more...

EDIT: And JulmaHerra states the problem more succinctly and clearly in the post above mine.