By this, I suppose you mean strive to be rich, and make the most of out what this world offers, correct?
Well, for a Jehovah's witness, this world is a facade of the one to come. This is Satan's time of rule, which is why the world is so bad. Not saying that there is nothing to enjoy in this life, or to have the the means to help yourself and others is a bad thing, but a true Christian will strive to do God's will above all. Jesus said at mat 6:24
“No one can slave for two masters; for either he will hate the one and love the other, or he will stick to the one and despise the other. YOU cannot slave for God and for Riches.
This was the famous Sermon on the Mount discourse he gave. He continued to go on and implore his disciples to stop being ANXIOUS over material things, and trust God to provide the things needed for you and your family.
Mat 6:31 So never be anxious and say, ‘What are we to eat?’ or, ‘What are we to drink?’ or, ‘What are we to put on?’ 32 For all these are the things the nations are eagerly pursuing. For YOUR heavenly Father knows YOU need all these things. 33 “Keep on, then, seeking first the kingdom and his righteousness, and all these [other] things will be added to YOU.
So Jesus highlights the most important aspect of a true Christian life, which is to seek the Kingdom of God above all things. And you cannot seek riches, and God's kingdom. To be rich in Satan's world requires all your time. Once you get there, you wanna stay there. So therefore your life will revolve around money, which cannot add a cubit to your life span.
He also said Jehovah knows what we need, and will provide. Now does this mean that one that serves God does not have to work? Of course not. Bible says if a man does not provide for his family, he is worse than a man without faith! But there is a difference between providing for your family, and chasing riches. Jesus was the ultimate proof of such. Here, he was a perfect man, can you imagine what our intelligence level would be if we were perfect? Yet instead of being some world reknown super scientist, Jesus chose to be a carpenter. Why? So he can spend most of his time preaching the good news of God's Kingdom, which will save lives in a way money never can.
In the earlier part of mathew 6, Jesus said
19 “Stop storing up for yourselves treasures upon the earth, where moth and rust consume, and where thieves break in and steal. 20Rather, store up for yourselves treasures in heaven, where neither moth nor rust consumes, and where thieves do not break in and steal. 21 For where your treasure is, there your heart will be also.
22 “The lamp of the body is the eye. If, then, your eye is simple, your whole body will be bright;
All these are reasons to simply serve God, which would store up treasures where it matters. Remember, Jesus also said, what does it benefit a man to gain the whole world, but lose his soul?
So this is why a Jehovah's witness will live as simply as they can, because the preaching work is more important. Also, what we don't have in financial resources, we make up for in other ways. Jehovah's witnesses were one of the first on the scene after Katrina for instance. A lot of brothers and sisters left their home areas to go help rebuild for those who lost homes and such. Many other examples world wide like this. So because you live simple, does not mean you cannot help