Hi @mys, very good question.
Depends on the RPC node that you connect to.

When you run a full rpc node there is an option called SKIP_BY_TX_ID. If it is set OFF you can query transactions by id. But not if it's set ON (and the node saves a lot of CPU time).

This steem explorer connects with https://api.steemit.com, which set this option ON. For this reason, I search transactions using the block ID.

It would be good to know if someone has a public node with this option OFF.