Regarding the title of the original post, Why don't more Christians tithe? Or at least give a little?
There's lots of reasons. A lot of Christians are tithing. A problem with our statistics is not with identifying how much Christians are giving, it is about who to include in the calculation—defining what it means to be Christian. The majority of the people in America who give themselves the label "Christian" are not followers of Jesus as demonstrated by their general choices and actions.
There are genuine followers of Christ who you can find all along the path between a worldly lifestyle (e.g., a new believer) and a Jesus-centered lifestyle. How easy it is to tithe is affected by what one understands of the material and spiritual value to tithing as well as how how much and what kind of faith someone has in God.
One reason some people do not tithe is that they are trying to do the minimum to be obedient to Jesus, rather than trying to love him with all their heart, soul, mind, and strength. (We all have this problem in varying degrees.) It is a reasonable interpretation of 2 Corinthians 9:7 to say that we are not strictly commanded to tithe (unlike baptism, for example). To be a cheerful giver, God needs to have given a person faith, and some do not yet have that faith. However, the effects of tithing are clear enough. Note the verse that precedes 2 Corinthians 9:7:
Remember this: Whoever sows sparingly will also reap sparingly, and whoever sows generously will also reap generously. (2 Corinthians 9:6, 1984 NIV)
The bottom line is not really "do I have to tithe?" because we don't "have to" do anything for God or anyone else. It is our choice (God freed us) and we will receive the consequences of our choice. The real question is more along the lines of, "to what degree do I see that it is priceless to invest all that I have and all that I am in God?"
Another reason some people do not tithe is because they do not perceive God's unchanging character (Malachi 3:6, Hebrews 13:8) in Malachi 3:7-15, but only see an old promise to farmers that has nothing to do with us. People think money is theirs and God wants 10% of it. However everything we have, including our money already belongs to God, and God calls not giving 10% of our increase "robbing" him (Malachi 3:8). In actual fact, not giving 100% of everything we are and have to God from one's heart is a form of rejection of God. This is probably the most important thing people should be working toward.
Bring the whole tithe into the storehouse, that there may be food in my house. Test me in this,” says the LORD Almighty, “and see if I will not throw open the floodgates of heaven and pour out so much blessing that you will not have room enough for it. (bold mine, Malachi 3:10, 1984 NIV)
Tithing for legalistic reasons (e.g., trying to be obedient to God in the way of the old covenant) is different than doing other things because of the Law, because God said "test me in this" about tithing in Malachi 3:11. As a result, God wants us to tithe even when our attitude is poor. But it isn't a direct part of being saved. Genuinely having entrusted ourselves to God is what is a direct part of being right with God (under both the Old Covenant and New Covenant).
It is important to note that if a person is trusting God even though they aren't tithing, they are personally better off in their personal relationship with God than if they are tithing grudgingly (but the potential recipients of the money are not better off).
And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him. (Hebrews 11:6, 1984 NIV)
However, just being pleasing to God is not all that is going on. There is still a spiritual battle going on (Ephesians 6:10-18). The spiritual world works in certain ways apart from what pleases God (Romans 5:13). We will still reap sparingly if we sow sparingly. So if you want the overflow of blessing that God spoke of in Malachi 3:10, fighting forward is still necessary (one example being tithing). For a little more info about that, see Deuteronomy 28 which is one expression of the framework of the spiritual battle we are in.