I think it's also a cost and availability issue.

Off-world items shipped in, perhaps from a long way away, will be much more expensive to buy and maintain than locally-produced goods. TL-15 kit requires TL-15 spare parts and consumables, as well as TL-15 trained technicians to look after it.  Some elite units might use better technology, but maintaining an entire TL-6 world army with TL-15 equipment is going to be difficult and expensive.

Also, there are a limited number of worlds producing this equipment. Obviously everyone would like the best toys, but perhaps not everyone will be able to get them. 

Add in the political dimension on inter-stellar arms trade (governments tend to have a say in who sells arms to who) and perhaps a world might not be able to get hold of advanced equipment (perhaps no one will sell to them because of their government policies).  

Also, politically a government might be under pressure to support local industries, or might not want to risk the supply being cut off because of the political whim of the selling government.