Why activity moved off the base layer
Ethereum's mainnet, often called Layer 1, is deliberately conservative: every transaction is processed and verified by the full network, which makes it highly secure but also limits how many transactions it can handle before fees rise. During periods of high demand, sending even a simple transaction on mainnet could cost more than the transaction itself was worth, which is a real problem if you want blockchains to be usable by ordinary people rather than just well-funded traders.
Layer 2 rollups solve this by processing transactions off the main chain and then posting a compressed summary, along with a cryptographic proof or fraud-detection window, back to Ethereum mainnet for final security. The result is transactions that cost a small fraction of mainnet fees while still inheriting most of mainnet's security guarantees. This isn't a marginal improvement, it's the reason most everyday on-chain activity, swaps, NFT mints, small transfers, has genuinely shifted to L2s rather than happening directly on mainnet.