It's not termed "raindrop", it's "airdrop" for your information. Well, we are on the same boat for that, they didn't clearly stated what are the conditions for this airdrop so it's expected we won't be able to tell if we're eligible or not...
There are clear criteria for awarding points for receiving airdrop, which are publicly available -
https://docs.arbitrum.foundation/airdrop-eligibility-distribution Similar criteria can be further used by other projects to calculate their airdrops in the future. Therefore, it is so important to know such criteria in order to be among the lucky recipients of airdrop in the future.