why is an organ made of different tissues

The simple answer is that organs need to be made of different tissues in order to best perform a designated job. Tissues are made of similar cells, and different tissues have different functions. Muscle tissue is for movement, epithelial tissue is for things like protection, nervous tissue is involved with sensing stimuli, and connective tissue connects different tissues together. Let's use the intestines as an example. The intestines move food through them via peristalsis (a wave-like muscle contraction). This means the intestines need muscle tissue, but it would be bad to have those digestive juices and enzymes coming into direct contact with muscle tissue, so the muscle is lined with epithelial tissue. Woven throughout these tissue layers is connective tissue holding it all together, as well as nervous tissue that monitors the food progress and controls the muscle contractions.
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