Because Labour and LibDems do not have a majority.
"Ah!" you cry. "But with the SNP and Plaid Cymru and all those other fringe parties, the Lib-Lab pact would have a majority of 14."
Well, up to a point, Lord Copper. [Emphasis mine.]
For such a government to have a majority it have to obtain at least the acquiescence of the SNP and Plaid Cymru. Both parties, however, as a matter of principle do not vote on English matters. This means that (assuming they can rely on the support of the DUP) the Conservatives would be able to defeat any measure brought forward by a putative anti-Tory coalition that applied only to England. On the other hand, even with a minority of seats in the UK as a whole the Conservatives would be able to rule England. This fact has been rather overlooked amid all the talk of deals and Parliamentary arithmetic, but it is really quite basic and obvious.
This means that it would be nearly impossible for the Lib-Lab coalition to get a majority at all, and almost impossible on devolved matters such as education, health, criminal justice, and more.
I just thought that was worth pointing out.