I am not a “liberal expert”, but am generally familiar.
The president can’t do anything they want, forever.
Biden used an executive order to enact a bunch of assistance programs during the end portion of covid. Included in that were things related to this topic.
Executive orders are temporary, and exist to allow the president to make fast adjustments to hopefully help people out or protect the country, on lots of topics. For example, if the president believes there is an imminent security risk, they can mobilize the military for a short time without Congressional approval.
If it seems like a topic that should be continued (like this one), Congress takes the issue up for a vote. Only Congress can make longer term budget decisions.
In this case Joe manchin campaigned on being aligned with this type of issue, and being aligned with Democrats on a variety of core positions. On several occasions in his career he has gone against this alignment at critically painful moments, and sided with republicans. Essentially not acting as he advertised. With Congress being closely split, his vote really matters.
So bidens temporary appropriations are now ending, and Congress didn’t continue them when it was expected they should be able to.
If Biden tried to just order it again, it would be struck down in courts due to this vote. This is an example of the balance of power between executive, representative, and judicial.