When making yogurt, why doesn’t bad bacteria grow as well

Yogurt is very sensitive and there are many possibilities here regarding why this recipe hasn't worked out, however the very main point that stands out for me is that you heated the yogurt to coagulate the protein.

This is not how it is done in traditional shanklish making. The yogurt is poured into an elongated container and gets covered and put on the floor. Then you need to sit on the floor and shake the container from front to back. Every ten minutes or so stop, uncovers the container and skims the butter which has formed on top. Continue doing so, until half of the butter is removed and the yogurt has become low in fat (partially skimmed). At this point you should be able to use the skimmed yogurt (known as shenineh) to heat it up and separate the curds from its water and make the cheese.

There are lots of ways of doing this, depending on exactly what you want in your final product. If you want an exact flavor and texture match to your specific brand of drinkable yogurt, you're going to have to try various things and experiment to see what works. Here are some issues to consider and options. (I'm assuming here that you're using store-bought yogurt as a base, since you post instructions that tell you how to produce a more drinkable style yogurt from scratch, and those should help if you were simply planning to make the yogurt yourself.)

• Be sure you buy yogurt without any added thickeners. Sometimes commercial yogurt adds substances to help maintain a thicker texture; obviously these will get in the way.
• Try blending your yogurt (or whisking or whatever you tend to use) to break up the set part as much as possible. For "homestyle" yogurts, this may be enough to thin them significantly.
• If you prefer your "drinkable yogurt" to have fruit, etc. in it, try adding the ingredients first and blending. The fruit juices may already thin the texture somewhat.
• If you still need it to be thinner, start by adding milk. If the result isn't "creamy" enough for your taste, add a mixture of milk and something with higher milkfat (light cream, half and half, etc.) next time. If the result isn't tangy enough for your taste, you could add some acidity, e.g., lemon juice could work well if it complements fruit flavors or something. Or, as an alternative for acidity, you could just dilute with another acidified milk product, like kefir or (cultured) buttermilk. (This last option would actually be an ideal choice to maintain the "fermented milk" flavor and tang.)

To go further with that last point, frankly in your situation I'd just skip the yogurt completely and use another acidified milk product with a texture closer to what you want. Kefir and cultured buttermilk both are produced by microorganisms similar to yogurt, they have similar acidity, and (in my experience) they taste pretty similar in a "smoothie" when blended with fruit or other ingredients to yogurt. On their own, the flavors will be somewhat different from yogurt, but the flavor of a yogurt/milk/juice/whatever concoction will taste different from the plain yogurt anyway. So, personally, I'd start with a thinner fermented milk product and go from there, unless you have some particular attachment to a specific yogurt brand or something.