Simple. Because it is inferior to the i3. It's slower, costs the same, uses more power.
Overall in games with a discrete GPU + applications, the A10-5800K is just 5% slower than i3 3220 in CPU performance and 1% slower than i3 2120.
Source
I am actually wondering if people bothered to read
any reviews on the web, or AT's review? A10-5800K is nearly as fast as i3 3220 in CPU tasks and blows the i3 away in GPU related benchmarks. Now you can go ahead and buy an i3 + $70 GPU and it'll be faster but it'll cost you $70 more and have higher power consumption than A10-5800K.
"At its stock settings, the company’s flagship A10-5800K is generally faster than Intel’s Ivy Bridge-based Core i3-3220/3225 in heavily-threaded applications and slower in x86-oriented tasks that only run on one core." ~
Tom's Hardware
In specific applications, the A10-5800K is actually faster than the i3 3200:
Source
The next step is to buy a Phenom or Pentium G processor and a discrete GPU. Phenom II X4 965 is not really faster than A10 piledriver at 4.2ghz so that's right away an inferior move with far higher power consumption. Pentium G620 on Newegg is $64, and it's slower than the A10-5800K in CPU performance. I've been linking for a while now in my CPU & GPU performance comparisons in VC&G forum that Bulldozer is actually performing faster in newer titles vs. Phenom II but people aren't paying attention to this.
The days when Phenom II was faster than Bulldozer for games are coming to an end and fast. Therefore, Phenom II is no way better than the A10 @ 4.2ghz.
The funny part is people are ignoring what the CPU is meant for. If you encode videos, do some heavy rendering and are buying this $122 CPU you are looking at the wrong product for your needs. As an HTPC and as a budget gaming system, nothing right now beats it.
The target market for this processor probably plays games that are very popular like Blizzard games and MMOs. This APU is perfect for them without needing a discrete GPU.
If you are going with discrete GPU route, the 2 cheapest 28nm options are either the lacklustre GT640 for $90-100 or the HD7750 for $80-90. Both of those GPUs cost nearly as much as the A10-5800K CPU. For the millions of people who mainly play WOW, Dota 2, Diablo 3, Portal 2 and other such graphically basic games, this Trinity CPU offers unbeatable price/performance, not to mention HD4000 graphics doesn't come on i3 CPUs.
Let's say you want to play the upcoming XCom: Enemy Unknown game, this APU will mop the floor with the i3 and Intel graphics. 5-10% lower CPU performance and up to 2x faster GPU performance of HD4000 graphics.