Are there any studies (fMRI scans, etc) showing why some people (supposedly) are more open to hypnotism?
Overall, while there are developing cognitive neuroscience theories of how hypnotic states are produced, there does not appear to be any known cognitive neuroscience basis for individual differences in hypnotic susceptibility based on a reasonable Google Scholar, Web of Science and Scopus search on the topic.

There is at least some evidence to suggest that highly hypnotic participants exhibit different EEG activity patterns (Sabourin et al., 1990). A later review of the hypnosis literature proposed an attentional explanation for the difference in hypnotic behavior for highs and lows (Crawford, 1994). They specifically propose that:


  highly hypnotizable persons (highs) possess stronger attentional filtering abilities than do low hypnotizable persons, and that these differences are reflected in underlying brain dynamics.


Finally, a still later study more specifically implicated the anterior cingulate cortex, the thalamus, and the ponto-mesencephalic brainstem in the production of hypnotic states (Rainville et al., 2002), but they make no mention of differences of highly and less hypnotically susceptible individuals. This seems to be a trend in the literature I reviewed, suggesting the concept may have been abandoned.

References


Crawford, H. J. (1994). Brain dynamics and hypnosis: Attentional and disattentional processes. International journal of clinical and experimental hypnosis, 42(3), 204-232.
Rainville, P., Hofbauer, R. K., Bushnell, M. C., Duncan, G. H., &amp; Price, D. D. (2002). Hypnosis modulates activity in brain structures involved in the regulation of consciousness. Journal of cognitive neuroscience, 14(6), 887-901.
Sabourin, M. E., Cutcomb, S. D., Crawford, H. J., &amp; Pribram, K. (1990). EEG correlates of hypnotic susceptibility and hypnotic trance: Spectral analysis and coherence. International Journal of Psychophysiology, 10(2), 125-142.