Reading data from the MCP3002
I thin the following is happening:

The MCP transmits 1 measurement per SPI transaction, which consists of activating chip select, reading data and deactivating chip select. To distinguish between different transactions, the time between deactivation and reactivation of chip select need to have a bit delay between them. Currently your code runs really fast, a few microseconds between the transactions at max.

This may also be connected to the MCP needing time to measure the analog signal.

I think there is some kind of race condition, so that the MCP does not have a new value to transmit, either because the MCP hasn't finished measurement yet, or because the delay between the transactions were just a bit too short, maybe a combination of both.

When the MCP is once in this situation, it will only send zeros at that point  (read chapter 5.1 in the datasheet. Last sentence in the left column).

You need to give the MCP more time between the transactions. I suggest using a small delay at the end of the loop() function. You can experiment with the actual value, I would guess, that you can go down to the microseconds range.