Why has it taken them so long? A bit of history may help us to understand.
When Microsoft released Office 2007, they updated key formats for saving files, including Word DOC, Excel XLS, and Powerpoint PPT files. These new file formats are effectively zipped XML text files (instead of the binary DOC, XLS, and PPT formats) and involve the adding of an "X" to the end of the old-style DOC, XLS, and PPT file extensions. Their introduction generated considerable controversy as Microsoft tried to get the ISO (the International Organization for Standardization) to certify them as a standard, a move which many viewed as an attempt by Microsoft to retain control of key file formats. Microsoft's Office Open XML (OOXML) formats were put forward to try to stop the growing use by governments and business of ODF, the OpenDocument format.
Although Microsoft succeeded in having their Office 2007 file formats certified, they have also yielded to pressure from governments and businesses that want to use ODF (hence, Microsoft included the ability to read and write ODF in Service Pack 2 for Office 2007).
Now that more and more people are using Office 2007 and saving to the default XML formats, this has created a need for other software suites to be able to open these files. OpenOffice.org, for instance, now has built-in Office Open XML functionality. However, beyond straightforward DOCX and XLSX files, things aren't so good outside the Microsoft Office universe. PPTX files in particular often lose a lot of formatting when opened in non-Microsoft programs.
According to PCPro, Google Docs has had to give in to keep up with the growing use of Microsoft's formats. Now, DOCX and XLSX files can be opened without too much trouble in Google Docs. PPTX files, however, remain problematic. As it's been some time since Microsoft's OOXML formats were certified, it may appear to be something of a wonder that it has taken Google so long to incorporate the ability to open some, if not all, of those formats. While Google has for some time made it possible to HTML-view DOCX and XLSX files in Gmail and Google Search, it is only now that the company is integrating more complete importing features for those formats into Google Docs.
The main reason behind Google's tardiness in this matter is likely a competitive desire not to acknowledge Microsoft's new file formats. In short, just as Microsoft resisted ODF in its own products, Google has been resisting recognising Microsoft's OOXML formats.