In IOTA timestamp accuracy cannot be trusted.

"The tangle is a graph with only a partial order structure, which makes it difficult (in fact, generally impossible) to establish the correct time order of transactions. Even if all transactions have timestamps on them, we cannot be sure that all these timestamps are accurate"

Source: http://iota.org/timestamps.pdf

In blockchain all blocks are organised into a linear sequence over time so this problem does not persist.

To make Smart Contracts on the Tangle possible, an "off-tangle" oracle will be implemented.

IOTA Foundation is actively working on Oracle platform for IOTA where the plan is also to enable true interoperability with all other Blockchain platforms.