[Sony Xperia M] Agu_Ra:
Because it is not goddamn regular boot image! It is designed to be flashed to FOTAKernel partition, which is inaccessible by fastboot (including the one included in flashtool). Are you able to boot your system after that? Normally you must be not. BTW, you are lucky that I'm now not going to insult you as I usually do to people ignoring provided information (I really love insulting people when I have a reason to, but now I'm not in that mood to do so, furthermore, I see curiosity in your question, I love curious people and am always glad to provide them needed knowledge if I'm able to). Instead, I'd like to bring some clarity regarding to the subject. So why one should not normally flash FOTAKernel image to the boot partition? Let's firstly talk about what is that mysterious FOTAKernel image. You'd not be surprised if I'll say that it is just regular recovery image like those made for devices having separate recovery partition. So what is recovery image? Technically, it is a boot image, the difference is inside. So let's at first learn about the things they have in common. They both are Linux kernel and ramdisk packaged together with some commandline parameters for bootloader to pass to kernel. After starting the device, bootloader loads kernel and ramdisk to the memory (RAM) with parameters defined in boot image's commandline and passes control to the kernel. Kernel initializes hardware, makes some of its low-level work and starts the first process called init from ramdisk (also called initial ramdisk or initramfs as it is loaded to memory only to initialize environment and mount the actual root partition, but in Android "initial" is omitted as Android's root filesystem remains mounted in RAM) as PID 1, itis empowered to do what the hell it wants to, but its responsibility is to get all userspace working: mounting partitions, creating device nodes, starting services etc. Recovery image is booted the same way, but it is designed to run completely in RAM and work even without any partition mounted. In many other devices (such as Nexus as reference Android implementation) boot decision is made by bootloader: it loads either regular boot image, which init configuration is designed to load complete Android system, or recovery image. which ramdisk contains recovery binaries and other needed stuff and which init is configured to load recovery binary (it is usually /sbin/recovery). But Sony's bootloader is designed to load only boot image (and stock FOTAKernel if bootloader is not unlocked), so we are using 2 stage boot as workaround (it's just like Russian matryoshka): recovery is actually loaded by regular boot image and, moreover, is packed inside it, boot decision is implemented not by bootloader, but by init itself: after kernel and ramdisk being loaded to memory init is started as PID 1, it shows familiar Sony logo, turns on purple led and waits for us to press a hotkey, so depending on hotkey pressed or not init is acting in one of three scenarios by unpacking the one of three 2nd-stage ramdisks to the root filesystem: if hotkey is not pressed, it upacks /sbin/ramdisk.cpio and regular boot is made, if VolDown is pressed, /sbin/ramdisk-recovery.cpio is unpacked and recovery from boot is booted, the most interesting is when VolUp is pressed: init checks for recovery image in FOTAKernel partition, if it is actually recovery image, its ramdisk is unpacked and recovery from FOTAkernel is loaded, the kernel in our recovery image is present (as its build is unified in Android build system), but it is ignored for we don't need it as we already have kernel loaded. So we are able to use recovery from the boot (in boot partition) or the actual recovery (from FOTAKernel partition, but technically we are able to boot it from anywhere, I've even tried to do so) image, and second one is persistent as it is not erased by the ROM updates, it is not interrupted until we explicitly want it to be so. So what happens if we flash recovery image to boot partition (e.g. by fastboot, Rashr etc.)? It actually acts like recovery image on the other devices loaded directly by bootloader. As it is not designed to load Android or alternative ramdisks at all (as opposed to our boot's ramdisk), it obviously doesn't provide a boot decision, so you'll end up being able to boot only to recovery.

You can check it yourself. Also if you are curious enough, you can unpack boot and TWRP images using Android Image Kitchen (you can find it on XDA) to see the difference between ramdisks and understand more clearly about what I'm talking about. /sbin/ramdisk-recovery.cpio from boot image ramdisk and the root of the recovery image ramdisk have actually the same structure.