Can anyone explain the reason behind this
The exact reason behind this is that the Google Ads algorithm(s) is "seeing" your account(s) as being created only to churn & burn, and hence you would get US$50 thresholds instead of US$350 thresholds and / or account suspensions.
And the exact solution to this is to experiment with different methods of creating the accounts. By methods, I am not referring to cards, IPs, proxies, etc, as these are just the resources to create the accounts. I am referring to the method of creating the accounts.
And remember, in 99% of cases of accounts suspended showing as due to suspicious payment, in reality it has nothing to do with payment, billing or card. I know people will not agree with this, but the same has always been attested by some other members here who are able to create working threshold GAds accounts. So, to believe it or not, that is your call.
Even with the best possible account creation method, the live success ratio is usually not 100%, but atleast 80% to sometimes 90% (which is good (thresholds US$350). You may take my suggestion or just ignore it : experiment with different methods of account creation, please.
And do not mind me mentioning one reality : nowadays, forget about sharing such working methods openly, fully & freely, people with such working methods do not even sell their methods, because nowadays it is highly monetizable and also prone to saturation always.
In reality this is the reason why people openly and freely share campaign, appeal, etc related things, but for account creation people usually share only some tips and hints.
Nonetheless, anyone can post a request in the "Want To Buy" section. Please pardon me for mentioning the reality, as I feel no one must have any incorrect expectations overall in life.