The idea is to place "only" as close as possible to what it's modifying. We're not saying that diabetes is "only surpassed" (as opposed to some other more extreme verb). We're saying that it is surpassed by two other causes, and only by those two causes. Take a look at these:
I only sing in the band. (The only thing I do in the band is sing. I may also sing in other contexts.)
I sing only in the band. (I don't sing outside of the band. I may also do other things in the band,
...
I only sing in the band. (The only thing I do in the band is sing. I may also sing in other contexts.)
I sing only in the band. (I don't sing outside of the band. I may also do other things in the band,
...
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