why can't I get the alpha? Is it invite only?

The level and time you spend in the program doesn't count for access to the Alpha group.

There is only a limited space for Alpha participants. If an existing Alpha participant leaves the Alpha group, or gets removed due to inactivity, Beta participants with a good activity record get an invitation.

It depends on the activity, and the quality of the feedback you sent while in the Beta ring if you get an invitation.

So there are two factors for getting an invitation.

You can't influence the first factor: Free space in the Alpha group. You will have to be patient and wait for someone to leave or be removed due to inactivity.

But the second factor, sending quality feedback and participating in the quests, will raise your chance to get an invitation in case a slot in the Alpha group becomes free.

So don't give up. Continue to give good feedback, and have patience.

There is only a limited number of free slots in Alpha/Alpha Skip Ahead.

If someone in Alpha/Alpha Skip Ahead leaves, or is removed due to inactivity, participants in Beta are invited to Alpha. However, Beta participant invitations are sent out to those, that leave valuable feedback, find bugs, and are most active in the beta
group.

So it's not how long you are in the program, but how valuable your input and bug reporting was in the past.

Added to the fact that invitations are only sent out if there is new room in Alpha, you might have to wait a while to get invited. Just provide good feedback and be patient.

I am insider. Based on your activities you can be invited in alpha or alpha preview ring. Only by someone from the staff. Ambassadors can't do that. Keep your surveys, ideas and reports clear and smart. Eventually, you will get an invitation. But beware.
To be an alpha means constant updates and bugs. I am using my second xbox for that. In some cases people had to send box to repair. Alphas are the first line of deffence against bugs. For safer community. As others have said you can become beta on your own
based on time and xp you have earned.

Alpha Ring Insiders are exclusive members the only way to get in is by getting an invite from a Microsoft Staff or Xbox Staff

You don't unless you join the insiders program which is on alpha skip and alpha only

Which both are invite only