Why does the proportion of transposable elements vary so much across species?
I can think of two factors that would influence this, but there are probably more.

1) How do transposable elements (TE) get into the genome in the first place?  The answer is both vertically and horizontally.  Vertical transmission needs no explanation.  Horizontal transmission of TE has been documented in several cases.  In some cases, infectious agents can transfer them between species individuals, for instance, viruses can package transposable elements in addition to their own DNA.

http://www.cell.com/trends/ecology-evolution/abstract/S0169-5347(10)00123-0

2) How efficient can the TE replicate?  For example, in the human genome there are a variety of different types of retrotransposons, but only the L1 element can replicate.  Others like Alu and SINEs parasitize their replication machinery and therefore replicate also but at much lower frequencies.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2884099/

So, the difference between species has to do with both of these and probably other factors.  I would imagine in the 2nd case, the selective pressures put upon a given species at any point in time, due to infections or inter-species competition, would surely impact the efficiency of TE replication because they are so important to genomic evolution and adaptation.