I agree with your analysis, the seasonal factor having definitely contributed to why one discipline was called up to the Olympics and not the other. However, and this is where I don't agree with your opening "No, not at all," the marketability of MTB absolutely played just as decisive a factor and, since you mentioned it, the fact that it first appeared on the Olympic agenda at Atlanta 96, in the US, is a demonstration of this.
I don't know what the situation with MTB was like outside the US in 96 and the years just prior, but MTB sales had been growing hugely domestically at the time and I imagine this was starting to become relatively true in Europe and Australia (the other markets that counted). Since most, if not all, brand and parts development was then "Made in the USA" there was a colossal insentive to develop the sport's market. And since at the IOC the US lobby is very strong, not forgetting the Atlanta location along with a rapidly growing MTB market and (as you emphasize) the sport takes place during the right season; everything was in place for the discipline to make its Olympic debut.
So, as I mentioned, "MTBing became popular because invented in the US, simple as that," is true (if nothing else, America knows how to develop markets). While such business interests and US influence at IOC doubtless got MTB an Olympics invitation at Atlanta in 96, despite the fact that as a sport it wasn't yet as popular as road cycling or CX. But the potential was there, and this was exploited at the right moment. By contrast, none of these factors, including the wrong season, played out to favor CX. CX bike sales weren't growing exponentially, nor were there large business interests to grow its market share, or a powerful IOC lobby to get the sport an Olympic slot.
I thus think these issues contributed equally as much to why MTB has become what it has today, whilst CX lost traction, despite more TV converge of the sport at Eurosport of late outside of Belgium and Holland.