For users willing to donate at all, the difference between (say) $2 and $5 might not be that much. Asking for more might therefore actually maximize the return.
All other users probably wouldn't donate regardless of whether it's $1 or less, so why bother considering them in your model. (edit) Offering an $1 option would just get you 0 of these users, and possibly lower the returns of the otherwise-$5-donaters above.
For users willing to donate at all, the difference between (say) $2 and $5 might not be that much. Asking for more might therefore actually maximize the return.
All other users probably wouldn't donate regardless of whether it's $1 or less, so why bother considering them in your model. (edit) Offering an $1 option would just get you 0 of these users, and possibly lower the returns of the otherwise-$5-donaters above.