This is pretty speculative on my part, but I think it comes down to a difference of valuations between the Front Office and Marcus. Probably by a fairly wide margin as well.
Through age 28, he actually stacks up fairly similar to Sonny Gray.
Gray: 900 IP, 3.66 ERA, 3.74 FIP, 3.1 BB/9, 7.9 K/9
Stroman: 734 IP, 3.80 ERA, 3.62 FIP, 2.6 BB/9, 7.3 K/9
Gray got 4 years and $38M. The Reds definitely got a bit of a discount there give his down season, but even if you increase it to 5 years and $55M taking into account him being a year younger, I can't really see Stroman taking that deal.