Because Bosmer and Khajiit have stealth focused racial passives. With the right builds, you can get hilariously close to enemies as either without being detected.
I play a Wood elf and even now i'm alright. Looking forward to stam increase, but honestly i was fine in both pvp and pve.
Skye Cloude - Sorc DPS, Master Crafter. Main, the bestest Lae Lenne - Templar Healer Trial grade. Dromede - Stamina Nightblade, she's a newb and doesn't know what she's doing V'oghatta - Stamplar pretending to be a tank Ulville Thonvella - aspiring Fire Mage, be careful around her fire sticks! Dromedaris - lost and not found. Named after a shoe, what else can you expect from her? A proper tank in her wildest dreams Swims-Naked - too pretty to grind, too silly to quest. Sun Flair - Dunmer Templar that can't spell for life. To bad she's too broke to afford a name change... Well, at least she's pretty...
I've made both. One Wood Elf, and one Kajiht. For being a thief the Kajiht has better sneaking ability. For all the other aspects of the game, I suggest the Wood Elf. Also, the Wood Elf is better for a two weapon system, which is what I did with mine. Bow, and Dual Weild. The only other option that is workable with the night blade is sword and board.... but sneaking around with them is kind of silly.
You have to factor in you will be wearing medium armor, and this build is not made for taking on any kind of tanking. fyi