HTC has always tended to trail competitors in battery sizes, since forever. People want a larger battery. It's as if they are continuously 6 months to a year behind the competition in battery sizes.
The Battery is non-removeable and smaller. They don't tend to have as good Power Management options as i.e. Sony or even Samsung at this point.
The only way to get around poor battery life on their embedded batteries is to find a charger or carry around a power pack. I'm pretty sure a spare battery is a lot more compact that that... You can get a Spare Battery + Charger form Samsung for the same price as some of those Power Packs and just swap the battery in their phones out, charge them both overnight (the spare comes with its own charger) and repeat the next day. Also, as the phone ages the battery loses capacity and you cannot restore that by buying a cheap replacement battery when the battery is embedded in the phone. This means a lot to people who actually keep their phones for a long time instead of upgrading every few months.
Other OEMs that are embedding batteries on their flagships tend to offer bigger sizes (3k mAh for the G2, for example).
2800 is only 200 mAh bigger than 2600, but it's actually ~8% capacity difference, which is not as trivial as a lot of people make it out to be.
Another thing is a lot of sites do rundown tests for battery life, but those don't even come close to telling the average user what kind of battery life to expect from the device in mixed usage. HTC's phones tend to underperform more than Samsung's, and a lot of people instantly blame the smaller batteries because of it. It can be a number of factors, but that is the most obvious one that sticks out to them.