Why do some trees have a life span, while some don't?
The answer to this could be that there are many factors contributing to the length of the life of tree species.

Climate: You can see that trees that have a reputation of becoming really old live in environments that have low moisture levels and much sunlight over the course of the year. 

For example, you can see that the most long-lived trees in America are located in California, where the temperatures are relatively high, while moisture levels are low. If you have one without the other trees don't tend to live that long. Cold climates make growth
harder for each individual, yet hot and moist climates tend to help trees grow easier, but also die easier. Think of the amazon. It is one of the richest forests in the world, yet the growth of bacteria and high competition in the areas of the equator make life a lot harder for the longest living and slowest growing species.

As you can see, the vast majority of the longest-living trees are located in California, or other regions of the same latitude!
http://en.wikipedia.org/wiki/List_of_oldest_trees

Other species and the life around it/Its Ecosystem: Individual trees tend to be less likely to be destroyed by a fire, as they are less likely to catch fire. At some instances, forest trees or dried-out grass can catch fire and pass it on to other individuals.
Microbes and bugs found in some parts of the world are known to their abilities to destroy trees!

Current size of the tree: Larger trees are generally less likely to die of causes like drought or damage to their trunks, as they have greatly extensive nets of roots and thick trunks and thus are able to, even partly, recover these incidences. 

If a rockfall for example causes damage to one side of the tree, some of the brunches will probably die, but the rest of them, that have their own network to the roots, will probably survive.

This means that luck is involved and if the tree is lucky enough to avoid dying for a certain amount of time, regular fluctuations in the environment do not kill it as they would kill a younger, less robust tree.

Biology of tree species: Debate is held as to whether tree cells have telomeres working the same way as they do in animals. Research published in 2001 suggests that telomeres don't work the same way, as plants developed without telomeres after six generations continue living and growing without problems. But even if they do use telomeres, they may use them in a different way. This is an area where research is still being conducted, though, and you may find controversies in the bibliography!
For example in the paper "Analysis of telomere length and telomerase activity in tree species of various life-spans, and with age in the bristlecone pine Pinus longaeva"
Barry E. Flanary &amp; Gunther Kletetschka which can be found in the rep

http://www.google.com/url?sa=t&amp;rct=j&amp;q=&amp;esrc=s&amp;source=web&amp;cd=2&amp;ved=0CCgQFjAB&amp;url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F7709949_Analysis_of_telomere_length_and_telomerase_activity_in_tree_species_of_various_life-spans_and_with_age_in_the_bristlecone_pine_Pinus_longaeva%2Ffile%2F9c96052bcf0a51be49.pdf&amp;ei=zzrCU9OcMIjV0QXmt4CwCQ&amp;usg=AFQjCNH7853fvZ8yneznSfIQQPsxtIDQag&amp;sig2=-PELK5RCQlAGTvQODoyZ2A&amp;bvm=bv.70810081,d.d2k

we can see that sometimes, short lived trees have longer telomeres than long lived ones!
(Look in the second paragraph of the section "Results"). This means that actually, nobody really knows!! It may be the telomeres, it may be something else, it may be a combination of factors.

http://www.madsci.org/posts/archives/2002-04/1019349924.Bt.r.html
(notice that this post has links to pages that are no longer available).

Distillation and the most important point: The way that the tree is adapted to its environment determines its longevity in it. Planting an olive tree in the amazon is a great way of making sure it will not survive! Some trees are better at surviving in a certain climate (even if it is at your own garden) simply because they have evolved to live there. 

That means that the trees probably don't have a defined lifetime in general, but the life expectancy is defined in each environment, depending on the circumstances!

This means that species we currently think of as short-lived can reach get really old: 
http://www.ncbi.nlm.nih.gov/pubmed/11295506

What some companies may have done is   try to combine characteristics of individuals through selective breeding - and yes, it works for trees as well!- to match the climate and terrain characteristics found in the certain country or region! Or they may have selected for resistance against the most common pests in your area!

I hope I answered you question adequately!