What Makes It Less Likely That Speciation Will Occur?

As far as I know, the main cause of speciation is isolation of a group for long periods, until this group has adapted to different conditions to those in which the original group lived, and thus changed so far that it can no longer interbreed with the original group. This is obviously more likely to happen with a larger population in the first place. Another major cause is when a number of (mostly food) resources are available, and different groups specialise in using different resources, adapting so they can do this more efficiently.

So speciation is less likely when the population has few opportunities to split up (eg, lives on flat or homogeneous territory, where it is hard for any group to become isolated or need to develop different characteristics) or where resources, however plentiful, are not very diverse, so there is no need to adapt to exploiting different ones.

I found a good overview of this subject here - of course you've probably seen it already.