As far as I know, there are at least two reasons. One is that despite the same name, a manufacturer might put a slightly different emulsion on the 120 for technical or manufacuring reasons, so the resulting development time might be different.(posted 8342 days ago)A second reason is that to minimise grain, 35mm is generally slightly under developed. Because enlarging ratios are smaller, MF doesn't need that so much, and can be developed to a slightly higher contrast. This presumably has some benefit in tonality. Sheet film negatives tend to be developed to a higher contrast again.
You could start with the same dev time as 35mm. Or maybe a tad more. There is a huge table of dev times on
http://www.digitaltruth.com/
but they come fom different sources, so might not be directly comparable.
Someone with more experience in MF than I will probably elaborate on the above.
Regards.........john stockdale