The thing is - they thought two doses was needed. Turns out likely one would be plenty, but to be SURE of that they would have to do more tests. The tests they have prove out a two dose routine. They can't tell you one is fine because they don't have data to support that yet, and honestly there is no real incentive for them to bother with it at this point.
It looks like one dose is very effective, and I can completely understand why people are balking at getting the second dose when you hear so many horror stories of feeling terrible afterward. If one dose gets you most of the way there, without side effects, why bother getting the second one and chancing the side effects unless you are high risk?
That is the argument with most younger people. The risk to them from the actual virus appears to be very small (though long term effects are unknown at this point), and the risk of short term side effects from the vaccine seems to be high from what I hear on the news and long term effects of the vaccine are unproven as well. Heck when you had the CDC telling people it was only proven effective for 3 months (because that was all the data they had at that point) then why would young people who are not at much risk get it at all?
Even now the CDC says it is effective for 6 months, and maybe longer but we don't know. For someone who is not at high risk from covid, there just isn't much incentive to be vaccinated IMO.
Maybe if they go through with the vaccine or negative tests are required for certain high risk activities (concerts, sports, etc) then there would be some incentive there. But the GOP is fighting that tooth and nail, so I don't expect it.
FWIW, I got my second shot last week. Based on news stories I was expecting to be sick the next day, and really dreading it, but I went through with it anyway. Happily my side effects were minimal.