Probably the same reason that <b> (bold) and <i> (italics) aren't used anymore. There's now a clearer delineation between document structure (described via tags) and style (described via CSS) than there was in the earlier HTML specs.
b and i are back though, but a little different ;)
> The b element represents a span of text to which attention is being drawn for utilitarian purposes without conveying any extra importance and with no implication of an alternate voice or mood, such as key words in a document abstract, product names in a review, actionable words in interactive text-driven software, or an article lede.
and
> The i element represents a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, transliteration, a thought, or a ship name in Western texts.