For two reasons-
1.) DPMS has a very bad reputation for quality and reliability. I say again, a very BAD reputation. I go to shooting courses and do my armorer update training, and I consistently talk to fellow cops and feds that have had problems with every DPMS they've ever laid their hands on. Right now, I'm seeing a malfunction rate of around 50%. I did a built a couple years ago with DPMS parts intentionally. I did an inspection on the parts and had to replace two small parts before installation because they were out of spec. I then had to modify the upper receiver and cut feed ramps into the upper receiver. A friend of mine is on one of the largest SWAT teams in the state, and they got (or were forcibly given, as they didn't make the decision themselves) new DPMS select-fire carbines. EVERY rifle malfunctioned right out of the box. The malfunctions were inconsistent, with some with failure to extract, some with failure to feed, some with failure to fire, and so forth. They had to replace every spring in every gun before the guns were able to be run reliably. Another agency got six new DPMS rifles, and the bolt gas rings shredded right out of it. The rifle went back to DPMS and was returned, and the rings shredded up and seized up the bolt again. Rifle was inspected and an armorer discovered that the bolts were not chromed inside, and the friction was shredding the gas rings. They put their CSO (community service officer- usually a college student) in a truck from the Minneapolis area and made him drive an hour up to St. Cloud to the DPMS factory and tell the people there to fix the problem and give them bolt carriers with chrome inside (which is proper mil-spec), and the situation repeated itself a third time. DPMS sent the rifles back with the CSO with NEW GAS RINGS only. The CSO didn't know any better. Finally, an armorer loaded up the guns and went up there a FOURTH time and demanded all new BCGs, or a refund. After some heated words and some issues, DPMS ultimately gave them new BCGs. The armorer then had to restake the carrier gas key screws, because DPMS cannot properly stake their gas key screws. I've never seen one properly staked. In fact, another metro agency here bought a couple new DPMS rifles for their squad cars, and during initial firing and testing of the rifles, several rifles had the BCGs literally fall apart IN THE GUN while firing. Gas keys were blown off and components on the bolt were damaged or missing.
With DPMS, you either get an "Okay" one, or a "Broken" one. The broken ones are typically only one or two parts away from being a functional gun, but you get to have the fun of figuring out what parts those are.
This does not include the numerous stories I hear of DPMS rifles that simply do not run, and they can't figure out why.
2.) that rifle kit is very basic, and antiquated. Specifically, you have an A1 upper that is worthless. It's an antiquated setup that should have died long ago. DPMS still wants to sell it, and it's idiotic. There is no reason to not get an A4/M4 upper receiver. The A1 has the fixed carry handle and A1 iron sights. You will have a lot of difficulty mounting any electronic sights on it. Additionally, you have a carbine without feed ramps in the upper receiver. DPMS offers an A3 upper receiver, which has the flattop that you can mount iron sights to, as well as any optic you choose. However, the A3 does not have feed ramps cut into it, and I can't figure out why DPMS still does not do this. Feed ramps improve feeding reliability. The A4 is the flattop upper with feed ramps in it. You can get them everywhere, as a lot of good places make them.
There are other options on the market that you should consider. What is your budget? How much is the MOST the you CAN spend?