Mechanical and normal is the default. Unless you have a specific reason to choose it, you're limiting yourself to a much smaller pool of possibilities and in general are considered worse, which is why they're not the default.

For mechanical vs optical: Mechanical was the default and still is because nothing else offers a good enough trade off to switch. Optical does offer some benefits, but wasn't enough for the entire market to swap. As far as I know, the most common selling points are: 1. They last longer because they use light instead of electrical contacts (While it might be technically true, in most cases people aren't replacing keyboards or switches because a switch dies from regular use. You're probably replacing it for other reasons, so in most cases switch lifespan shouldn't affect your decision IMO. If you're able to swap switches anyway whether because you can solder or it's hotswap, it's not really a big deal to replace dead switches. Even if you're not going to fix dead switches, if you used your keyboard down to actually wear out a switch, you're probably more than happy with how long it lasted anyway) 2. They actuate faster (Sometimes, but not always as this just depends on the actuation distance. Some mechanical switches, often called speed switches, offer shorter actuation points, but it isn't always desired since it can also increase the frequency of fat finger mistakes) 3. They have less latency (I don't know too much, but apparently using optical switches can offer lower latency than mechanical switches because they have to work around something called debounce. So while it technically can reduce latency, I haven't seen anything regarding the difference in latency and whether it's an amount that someone would or could even notice in the context of everything else that adds latency.)

As for low profile, I don't know if you mean low profile cases or low profile switches. Low profile cases is almost purely an aesthetic choice, one that most people don't like. Me personally I used to use low profile cases exclusively because I used my palm to press left ctrl. I've since swapped my left ctrl to where caps lock normally is and have moved away from low profile cases. They're super limited in choice, so unless you have a specific desire, it's generally not recommended. Unless you REALLY want to see your switches from the side or something (even then, just get something with a clear case).

If you're talking low profile switches, I don't have personal experience in these. As far as I know, most low profile switches require low profile keycaps as well, though I believe I have seen cherry compatible low profile switches. You'd only really look for low profile switch keyboards if you really wanted it for it's thinness. Nobody picks and uses low profile switches for any other reason. (Basically they offer nothing other than its lack of thickness, but you are giving up a lot such as a lot of options, lower travel time, etc.)