I'm really not sure if the OP was including London and Dubai to say that they're also limited, or if they were included as examples with much better presence.

OP, if it's the former, I'm really not sure you're going to get much support. 55 hotels is a lot, basically allowing any traveler to find a Marriott property in their preferred area, taste, and price (keeping in mind London's expensive on the whole).

If it's the latter, well, as your own research points out, they're the largest chain in the world. Just about anywhere travelers want to go, they're going to have an option-or options, in the Marriott family. Choosing one specific destination, and deciding that the number of choices doesn't meet your standard, is certainly your prerogative, but it's fairly arbitrary.

There can always be an anomoly-whether it's because Marriott hasn't succeeded there in the past, or they have difficulty with property owners/managers, or even just the city tending to have a higher concentration of independent properties. None of that changes the fact that by and large, Marriott's footprint is massive.

It's worth noting, the article you cite, lists the next largest chains as well. Here's how they stack up in Rome:

Hilton-8 hotels
Accor-13 hotels
Best Western-21 hotels
Wyndham-1 hotel

So, on average, they're a touch smaller than Marriott.
But, looking a bit closer....the furthest Marriott is 7.6 miles from the city center. Using that same metric, Accor drops down to....7 hotels. Best Western drops down to 14.

Other large chains: Hyatt-7 properties; Choice-6 properties; IHG-5 properties

So really, it seems that the takeaway here is, Rome has a lot of smaller brands or independents, but within the realm of loyalty programs, Marriott is still one of the best represented.