Why does a linear blockchain limit transaction throughput?

A linear blockchain can only accept one block per level, which creates a hard ceiling on how fast it can process transactions. Because every block has exactly one parent and the chain forms a single sequence, the network can agree on only one block at each position. Any competing blocks are orphaned — rejected along with every transaction inside them. This means that simply producing blocks faster does not proportionally increase throughput, because the extra blocks just get discarded. For a beginner, this bottleneck is the core limitation that newer blockDAG designs were built to address.