States with large populations such as New York, California and Illinois may favor strong gun control laws but many with much smaller populations strongly support gun rights. So while the majority of Americans might support an AWB, the majority of states might not.
But some Democrats in the Senate do come from smaller Red states. The current push for the AWB by some in the Democratic Party might endanger the seats of pro-gun Senate Democrats in these states as it could rally Democratic voters to replace them with pro-gun control candidates in upcoming primaries who would not enjoy the advantage of incumbency when they run against a Republican candidate.
Consequently our party could lose seats in the Senate. Most Republicans in the Senate are known to support gun rights and proving that they do by forcing a vote will not do as much damage to them as it will to Democrats.
Currently there are 53 Democratic Senators, 45 Republicans and 2 Independents. Senators who support gun rights from small states can offset the Senators from states with larger populations so the fact that the majority of Americans wish to see an AWB pass is irrelevant in the Senate. States with large populations do not hold more power than states with low populations in the Senate.
Remember that it isn't t a Senator's job to bow to the wishes of the majority of people in the nation but to represent the views of the voters who elected him. If the majority of voters in his state that sent him to office oppose an AWB, why should he vote for one.