From my understanding it's mostly head-of-the-line blocking.

HTTP/2 was introduced to solve head-of-the-line blocking on the application level (one HTTP/1.1 connection could only handle one request at the time, barring poorly supported pipelining)

TCP itself also only allows one stream of data to flow at the time (in each direction) - in case of poor network quality you are going to lose some TCP segments, and if you fill in the receiver's window, the transfer might halt until retransmission succeeds (this includes ALL streams within HTTP/2). Using UDP allows you to circumvent that.