How could one say that older operating systems are more vulnerable?

There is one flaw in your premise that since those malware ran on windows 10 but not on 98 and hence windows 98 is safe. By that logic, I can say that my shell script cleared the contents of a linux box but not windows and hence windows is more secure.

Edit: Modern operating systems are the primary targets for attackers as most of the users use them. If the features used in the attack are not supported by older operating systems, the attack will not run on them. It doesn't make them secure. The attacker is just not bothered about them.

The main reason older operating systems are not considered secure is that if a vulnerability is found on them, the vendor will not bother providing a patch for it. Their support has been ended.

All other reasons are secondary.

You need to make one adjustment to your conclusion:

So, I see Windows 10 gets more infected than Windows 98

Should be

So, I see Windows 10 gets more infected than Windows 98 by the programs I tried

You cannot make general comments about Windows based on the small sample set of software and malware you tried, especially since you indicate that there was incompatibility issues (which seems to indicate that you tried software bound to one OS).