When HTC released the Nexus One with Google, and then a bunch of other devices based on mostly the same hardware, HTC became the favorite company of the custom ROM developer community. That was because Motorola's devices were not as developer-friendly and Samsung hadn't yet launch the Galaxy S flagship device.
But due the popularity of Samsung's Galaxy devices, and because Samsung has always released the kernel source code shortly after the shipping of the devices, custom ROM developers have become a lot more fond of Samsung than of HTC. While the people who want to port custom ROMs on their devices may not be that great in number, I the openness of a device to hacking and tweaking does create a significant influence in the buying decision of many people. HTC is certainly not in a position to ignore any sales right now.
HTC started offering unlock codes for their devices a while back, but without the source code to make it easy for developers to make ROMs for the devices, it's all rather pointless. HTC has been keeping the kernel source code, which they need to release within weeks after starting shipping the devices, for a lot longer than they should, which is usually between 90-120 days.
This has caused a bit of an outrage online, with several people claiming to sue HTC over copyright infringement because they are bound by the GPL license to release their code and modifications a lot earlier than they are releasing it right now. The same people have started a Change.org petition to convince HTC publicly to agree to releasing their source code sooner — as soon as they release their devices.
Their goal for the signatures is 7,000, and they've already reached 1,615 as of writing. If you think that this is an important issue, especially if you're a custom ROM user on HTC devices, then you should be signing the petition right now at the source link.