Google says Thunderbird is a "less secure app"

I read/get/send my mails with Thunderbird. I use SSL for auth.

I was forced to make a setting in my Google account "Allowing less secure apps to access your account".

Google doesn't say why it's less secure.

Google Support

Some examples of apps that do not support the latest security standards include:

Some Desktop mail clients like Microsoft Outlook and Mozilla Thunderbird

Why Thunderbird is "less secure"?