Why is expenses excluded from balance sheet?

Answer:The balance sheet shows the resources (assets) on the debit side, and the funding of the resources (equity and liabilities) on the credit side, at a point in time.

Expenses (just like revenues) are measured over some period. As an analogy, when you open the fridge, you can count the number of bottles of beer that you have ("assets", what you have at some point), but you won't see how many bottles of beer you drank last month ("expenses", measured over a period).