Control of the intellectual property. Intel already controls what's in most desktops since Apple ditched PowerPC. No competition there across the globe. Tablets and mobiles were the challengers eating into that market share. ARM dominates there with its I.P. in almost everything that's mainstream with over 10 major companies licensing the I.P. for their own chips. If Intel owned that, they'd control the I.P. behind most of their rivals. Finally, there's the server situation where, although more ISA competition, x86 still dominates.
Letting Intel have ARM would certainly be an antitrust situation given the sheer dominance Intel's I.P. would have at that point.
