In this post, I will not focus on the titles itself, but more looking on the server side of the topic and how the different technical approaches might have an influence to the player counts.
This post is more an commentary than a proper statistic. Just by two cents, basically.
While all of the titles require servers, they differ in the way the servers are deployed. On the examined titles, they can be divided into three groups. These groups are "dedicated hosting", "self hosted" and "on premise".
Let's define them quickly before we continue.
This group has third-party companies and individuals offering hosting in a (semi-)automated way, e. g. management using a web interface. This approach allows non-technical users to get servers running, but they need to approach third party companies or individuals for that, while the hosting itself is not free. A disadvantage can be that on shared hosting approaches overbooking might occur.
Note: NOT counting into this groups is hosting where people do by hand updating of server software. Titles with both self-hosting and dedicated hosting are included in the group for dedicated hosting.
For titles in this group no hosting is available. The users need to host an base system by themselves and install the dedicated server on top of it. In this approach, the user is required at least basic knowledge about networking, operating systems to get the server running. Especially for at home hosting, additional limitations may occur based on ISP side limitations. Additional costs for infrastructure and operating systems are required.
Basically, this option is not an really option if you don't have proper knowledge to manage the server system itself.
The developer of this titles offering hosting on their own infrastructure. The user does not need to manage an base server on their own. For the end user, this is the most convenient and easy way to get a server running.
Influence of hosting approaches on the player base
On a player base of approx. 25000 players (as of 2nd october 2021), following distribution of approaches can be monitored. Player counts are based on the values from https://steamdb.info/, while iRacing player count was the displayed user count in the start screen. Included titles are PC titles, console titles are excluded. Also not included are titles considered as "arcade" titles.
Taking a careful comparision
Compared to average, on premise titles have 47 percent higher player counts. This is quite similar to titles with dedicated hosting, which are located 43% above the average player count. In contrast, self-hosted titles bind -224% less players compared to the average player count as of 2nd october 2021.
Bold statement: In simracing, self hosting failed. It failed hard. Users do not want to manage the server itself. They want easy to understand and quick solutions to get servers running. On premise and dedicated hostings manage to bind significantly more players than the ones running in a self-hosted concept. For titles with dedicated hosting, a better featureset for automation could be the foundation for their usage within third party server hosting.
Bold statement II: The existence of a proper server side infrastructure might be beneficial for the player count of a title.