How are material possessions not as important than true love?
I think that the question might be better read with the "not as important" inclusion. No doubt that material possessions fade in comparison to true love, but the reality is that material possessions do have and hold importance. I think that one, as previously noted, will experience different opinions on this particular topic. Emotional connection and spiritual sharing is extremely important. Yet, the reality is that some do not have the chance nor opportunity to experience this connection with another. It is important to ensure that individuals who do not share this or cannot experience this with another are not written off nor considered less than others for this condition. If an individual cannot find true love, perhaps then, material possessions might bear importance and might be important. I would not be so inclined to dismiss material possessions for individuals in certain predicaments. However, I do believe in the power of spiritual and truly valid expressions of love. If compelled to choose, I would select true love, but I am not sure that this is something others would share nor should be compelled to share.
Obviously this is a matter of opinion and experience.
I personally believe humans were created in order to have relationships with other humans - after all, our ability to relate - on emotional levels - is one thing that separates us from animals. Ultimately, one of the deepest emotions two people can experience together is true, unabashed, unconditional love. From experience I can tell you that it is a feeling that no amount of personal possessions can imitate - no matter their worth.
You can find this theme throughout literature as well - the idea that all material things will eventually fade away - deteriorate, lose their value, etc. - but true love lasts forever.
Consider when people are on their death beds - what do they wish to be surrounded by? Their big house, car, and a print out of their stock portfolio? Obviously not. They wish to be surrounded by the people they love most.
I do not know if you are asking this in general, or with reference to the story.
With reference to the story, material possessions are clearly less important to Jim and Della (at least at the end of the story) than true love is. We can see this because of the way that they react to the surprise ending. At the end of the story, they have both lost their most prized material possessions. Even so, they are still happy, they are still in love with one another.
This shows that they value love more than possessions. Della had seemed to worry for a while that Jim would be angry, or would not find her attractive anymore. But, as it turned out, he was still just as much in love with her (and she with him).