The primary deciding factor is always security. You simply cannot use any small vendor because of the physical security (or the lack thereof). Unless of course you do not care about security. If a red team can just waltz into you DC and connect directly to your infra is it game over for some businesses. You can easily do this with most vendors.
The secondary deciding factor is networking. Most traditional co-los have very limited understanding of networking. A CCIE or two can make a real difference. Unfortunately those guys usually work some bigger companies.
The third deciding factor air conditioning and electricity considerations. Worst case you are facing an OVH situation. https://www.datacenterdynamics.com/en/opinions/ovhclouds-dat....
(It is really funny, because I have warned them that their AC/cooling solution is not sufficient, and they explained to me that I am wrong. I was not aware of the rest (wooden elements, electricity fuckups, etc.)
"""During the year, an article in VO News by Clever Technologies claimed there were flaws in the power design of the site, for instance that the neighboring SBG4 facility was not independent, drawing power from the same circuit as SBG2. It's clear that the site had multiple generations, and among its work after the fire, OVHcloud reported digging a new power connection between the facilities.""")
The fourth would be probably pricing. TCO is one consideration, after you made sure that the minimum requirements are met, but only after.
So based on the needs somebody can choose wisely, based on the ___business requirements___. For example, running an airline vs running a complex simulations have very different requirements.